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CBSE Class 12 Maths Chapter 13 Probability – NCERT Solutions Exercise 13.3 [2025-26]

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Download Free PDF of Probability Exercise 13.3 NCERT Solutions for Class 12 Maths

As you start tackling Class 12 Maths Chapter 13, Probability, you’re entering a key section of the CBSE syllabus. Exercise 13.3 focuses on conditional probability and event independence—concepts that students often find tricky before exams. Here, you get step-by-step answers for probability class 12 ncert solutions, so you don’t have to worry about missing crucial logic or hidden formulas.

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The weightage for Probability in board exams is significant, typically carrying up to 8 marks. Every mark matters, and mastering exercise 13.3 can directly impact your overall score. Using exact methods, such as the multiplication law and clear stepwise solutions, aligns your preparation with real board expectations and solves the query “exercise 13.3 class 12.”


Vedantu offers these NCERT Solutions for Class 12 Maths Chapter 13 Exercise 13.3 with full reliability, so you can revise with confidence and focus on scoring your best in Probability this year.

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Access NCERT Solutions for Maths Class 12 Chapter 13 - Probability

Exercise 13.3

1. An urn contains 5 red and 5 black balls. A ball is drawn at random; its colour is noted and is returned to the urn. Moreover, 2 additional balls of the colour drawn are put in the urn and then a ball is drawn at random. What is the probability that the second ball is red?

Ans: Given the urn contains 5 red and 5 black balls.

Let a red ball be drawn in the first attempt.

Then, probability of drawing a red ball \[=\dfrac{5}{10}=\dfrac{1}{2}\]

Now, we add two more red balls. Therefore the urns contain 7 red and 5 black balls.

Probability of drawing the second red ball \[=\dfrac{7}{12}\]

Let a black ball be drawn in the first attempt.

Then, probability of drawing a black ball \[=\dfrac{5}{10}=\dfrac{1}{2}\]

Now, we add two more red balls. Therefore the urns contain 5 red and 7 black balls.

Probability of drawing the second red ball \[=\dfrac{5}{12}\]

Therefore, total probability of drawing second ball as red is

$\dfrac{1}{2}\times \dfrac{7}{12}+\dfrac{1}{2}\times \dfrac{5}{12}=\dfrac{1}{2}\left( \dfrac{7}{12}+\dfrac{5}{12} \right)$

$=\dfrac{1}{2}\times 1$

$ =\dfrac{1}{2}$

2. A bag contains 4 red and 4 black balls, another bag contains 2 red and 6 black balls. One of the two bags is selected at random and a ball is drawn from the bag which is found to be red. Find the probability that the ball is drawn from the first bag.

Ans: Let A be the event of getting a red ball.

Let \[{{E}_{1}}\] and \[{{E}_{2}}\] be the events of selecting the first bag and second bag respectively.

\[P({{E}_{1}})=P({{E}_{2}})=\dfrac{1}{2}\]

\[\Rightarrow P(A|{{E}_{1}})=\]P(drawing a red ball from first bag) \[=\dfrac{4}{8}=\dfrac{1}{2}\]

\[\Rightarrow P(A|{{E}_{2}})=\]P(drawing a red ball from second bag) \[=\dfrac{2}{8}=\dfrac{1}{4}\]

The probability of drawing a ball from the first bag, 

given that it is red, is given by \[P({{E}_{1}}|A)\]

By using bayes theorem, we obtain

$P({{E}_{1}}|A)=\dfrac{P({{E}_{1}})P(A|{{E}_{1}})}{P({{E}_{1}})P(A|{{E}_{1}})+P({{E}_{2}})P(A|{{E}_{2}})}$

$=\dfrac{\dfrac{1}{2}\times \dfrac{1}{2}}{\dfrac{1}{2}\times \dfrac{1}{2}+\dfrac{1}{2}\times \dfrac{1}{4}}$

$=\dfrac{\dfrac{1}{4}}{\dfrac{1}{4}+\dfrac{1}{8}}$

$=\dfrac{2}{3}$

3. Of the students in a college, it is known that 60% reside in hostel and 40 %are day scholars (not residing in hostel). Previous year results report that 30 %of all students who reside in hostel attain A grade and 20 %of day scholars attain A grade in their annual examination. At the end of the year, one student is chosen at random from the college and he has an A grade, what is the probability that the student is hostler?

Ans: Let \[{{E}_{1}}\] and \[{{E}_{2}}\] be the events where the student is a hostler and a day scholar respectively.

Let A be the event that the chosen students get a grade A.

\[P({{E}_{1}})=60%=0.6\]

\[P({{E}_{2}})=40%=0.4\]

\[\Rightarrow P(A|{{E}_{1}})=\]P(student getting an A grade is a hostler) \[=30%=0.3\]

\[\Rightarrow P(A|{{E}_{2}})=\]P(student getting an A grade is a day scholar) \[=20%=0.2\]

The probability that a randomly chosen student is a hostler, given that he has an A grade, is given by \[P({{E}_{1}}|A)\]

By using Bayes theorem, we get,

$P({{E}_{1}}|A)=\dfrac{P({{E}_{1}})P(A|{{E}_{1}})}{P({{E}_{1}})P(A|{{E}_{1}})+P({{E}_{2}})P(A|{{E}_{2}})}$

$=\dfrac{0.6\times 0.3}{0.6\times 0.3+0.4\times 0.2}$

$=\dfrac{0.18}{0.26}$

$=\dfrac{9}{13}$

4. In answering a question on a multiple choice test, a student either knows the answer or guesses.  Let \[\dfrac{3}{4}\] be the probability that he knows the answer and \[\dfrac{1}{4}\] be the probability that he guesses. Assuming that a student who guesses at the answer will be correct with probability \[\dfrac{1}{4}\] . What is the probability that the student knows the answer given that he answered it correctly?

Ans: Let \[{{E}_{1}}\] and \[{{E}_{2}}\] be the events in which the student knows the answer and the student guesses the answer respectively.

Let A be the event that the answer is correct.

\[P({{E}_{1}})=\dfrac{3}{4}\]

\[P({{E}_{2}})=\dfrac{1}{4}\]

\[\Rightarrow P(A|{{E}_{1}})=\]P(student answer correctly, given he knows the answer) \[=1\]

\[\Rightarrow P(A|{{E}_{2}})=\]P(student answer correctly, given that he guessed) \[=\dfrac{1}{4}\]

The probability that the student knows the answer, given that the answer is correct, is given by \[P({{E}_{1}}|A)\]

By using Bayes theorem, we get,

$P({{E}_{1}}|A)=\dfrac{P({{E}_{1}})P(A|{{E}_{1}})}{P({{E}_{1}})P(A|{{E}_{1}})+P({{E}_{2}})P(A|{{E}_{2}})}$

$=\dfrac{\dfrac{3}{4}\times 1}{\dfrac{3}{4}\times 1+\dfrac{1}{4}\times \dfrac{1}{4}}$

$=\dfrac{\dfrac{3}{4}}{\dfrac{3}{4}+\dfrac{1}{16}}$

$=\dfrac{3}{4}\div \dfrac{13}{16}$

$=\dfrac{12}{13}$

5. A laboratory blood test is 99%effective in detecting a certain disease when it is in fact, present. However, the test also yields a false positive result for 0.5%of the healthy person tested (that is, if a healthy person is tested, then, with probability0.005, the test will imply he has the disease). If 0.1percent of the population actually has the disease, what is the probability that a person has the disease given that his test result is positive?

Ans: Let \[{{E}_{1}}\] and \[{{E}_{2}}\] be the respective events that a person has a disease and a person has no disease.

\[P({{E}_{1}})=0.1%=0.001\]

Since \[{{E}_{1}}\] and \[{{E}_{2}}\] are events complementary to each other

\[P({{E}_{2}})=1-P({{E}_{1}})=1-0.001=0.999\]

Let A be the event that the answer is correct.

\[\Rightarrow P(A|{{E}_{1}})=\]P(result is positive given the person has disease) \[=99%=0.99\]

\[\Rightarrow P(A|{{E}_{2}})=\]P(result is positive given that the person has no disease) \[=0.5%=0.005\]

The probability that a person has a disease, given that his test result is positive, is given by \[P({{E}_{1}}|A)\]

By using Bayes theorem, we get,


$P({{E}_{1}}|A)=\dfrac{P({{E}_{1}})P(A|{{E}_{1}})}{P({{E}_{1}})P(A|{{E}_{1}})+P({{E}_{2}})P(A|{{E}_{2}})}$

$=\dfrac{0.001\times 0.99}{0.001\times 0.99+0.999\times 0.005}$

$=\dfrac{0.00099}{0.00099+0.004995}$

$=\dfrac{0.00099}{0.005985}$

$=\dfrac{22}{133}$

6. There are three coins. One is two headed coin (having head on both faces), another is a biased coin that comes up heads 75% of the time and third is an unbiased coin. One of the three coins is chosen at random and tossed, it shows heads, what is the probability that it was the two headed coin?

Ans: Let \[{{E}_{1}}\], \[{{E}_{2}}\] and \[{{E}_{3}}\]be the events of choosing a two headed coin, a biased coin, and an unbiased coin respectively.

Let A be the event that the coin shows heads.

\[\therefore P({{E}_{1}})=P({{E}_{2}})=P({{E}_{3}})=\dfrac{1}{3}\]

A two-headed coin will always show heads.

\[\Rightarrow P(A|{{E}_{1}})=\]P(coin showing heads, given that it is two-headed coin) \[=1\]

Probability of getting heads, given that the coin is biased \[=75%\]

\[\Rightarrow P(A|{{E}_{2}})=\]P(coin showing heads, given that the coin is biased) \[=\dfrac{75}{100}=\dfrac{3}{4}\]

The third coin is unbiased hence the probability of getting heads is \[=\dfrac{1}{2}\]

\[\Rightarrow P(A|{{E}_{3}})=\]P(coin showing heads, given that the coin is unbiased) \[=\dfrac{1}{2}\]

The probability that the coin is two-headed, given that it shows heads, is given by \[P({{E}_{1}}|A)\]

By using Bayes theorem, we get,

$P({{E}_{1}}|A)=\dfrac{P({{E}_{1}})P(A|{{E}_{1}})}{P({{E}_{1}})P(A|{{E}_{1}})+P({{E}_{2}})P(A|{{E}_{2}})+P({{E}_{3}})P(A|{{E}_{3}})}$

$=\dfrac{\dfrac{1}{3}\times 1}{\dfrac{1}{3}\times 1+\dfrac{1}{3}\times \dfrac{3}{4}+\dfrac{1}{3}\times \dfrac{1}{2}}$

$=\dfrac{\dfrac{1}{3}}{\dfrac{1}{3}\left( 1+\dfrac{3}{4}+\dfrac{1}{2} \right)}$

$=\dfrac{1}{9}\div 4$

$=\dfrac{4}{9}$

7. An insurance company insured 2000 scooter drivers, 4000 car drivers and 6000 truck drivers. The probability of accidents are 0.01, 0.03 and 0.15 respectively. One of the insured persons meets with an accident. What is the probability that he is a scooter driver?

Ans: Let \[{{E}_{1}}\], \[{{E}_{2}}\] and \[{{E}_{3}}\]be the events that the driver is a scooter driver, a car driver, and a truck driver respectively.

Total number of drivers = 2000 scooter drivers + 4000 car drivers + 6000 truck drivers = 12000 drivers

\[\therefore P({{E}_{1}})=\dfrac{2000}{12000}=\dfrac{1}{6}\]

\[P({{E}_{2}})=\dfrac{4000}{12000}=\dfrac{1}{3}\]

\[PP({{E}_{3}})=\dfrac{6000}{12000}=\dfrac{1}{2}\]

Let A be the event that the person meets with an accident.

\[\Rightarrow P(A|{{E}_{1}})=\]P(scooter driver met with an accident) \[=0.01=\dfrac{1}{100}\]

\[\Rightarrow P(A|{{E}_{2}})=\]P(car driver met with an accident) \[=0.03=\dfrac{3}{100}\]

\[\Rightarrow P(A|{{E}_{3}})=\]P(truck driver met with an accident) \[=0.15=\dfrac{15}{100}\]

The probability that the driver is a scooter driver, given that he met with an accident, is given by \[P({{E}_{1}}|A)\]

By using Bayes theorem, we get,

$P({{E}_{1}}|A)=\dfrac{P({{E}_{1}})P(A|{{E}_{1}})}{P({{E}_{1}})P(A|{{E}_{1}})+P({{E}_{2}})P(A|{{E}_{2}})+P({{E}_{3}})P(A|{{E}_{3}})}$

$=\dfrac{\dfrac{1}{6}\times \dfrac{1}{100}}{\dfrac{1}{6}\times \dfrac{1}{100}+\dfrac{1}{3}\times \dfrac{3}{100}+\dfrac{1}{2}\times \dfrac{15}{100}}$

$=\dfrac{\dfrac{1}{6}\times \dfrac{1}{100}}{\dfrac{1}{100}\left( \dfrac{1}{6}+1+\dfrac{15}{2} \right)}$

$=\dfrac{1}{6}\div \dfrac{104}{12}$

$=\dfrac{1}{52}$

8. A factory has two machines A and B. Past record shows that machine A produced 60% of the items of output and machine B produced 40% of the items. Future, 2% of the items produced by machine A and 1% produced by machine B were defective. All the items are put into one stockpile and then one item is chosen random from this and is found to be defective. What is the probability that was produced by machine B?

Ans: Let \[{{E}_{1}}\] and \[{{E}_{2}}\] be the respective events of items produced by machines A and B.

\[\therefore P({{E}_{1}})=60%=\dfrac{3}{5}\]

\[P({{E}_{2}})=40%=\dfrac{2}{5}\]

Let A be the event that the produced items were found to be defective.

\[\Rightarrow P(A|{{E}_{1}})=\]P(product is defective, given that machine A produced) \[=2%=\dfrac{2}{100}\]

\[\Rightarrow P(A|{{E}_{2}})=\]P(product is defective, given that machine B produced) \[=1%=\dfrac{1}{100}\]

The probability that the randomly selected items was from B, given that it is defective, is given by \[P({{E}_{2}}|A)\]

By using Bayes theorem, we get,

$P({{E}_{2}}|A)=\dfrac{P({{E}_{2}})P(A|{{E}_{2}})}{P({{E}_{1}})P(A|{{E}_{1}})+P({{E}_{2}})P(A|{{E}_{2}})}$

$=\dfrac{\dfrac{2}{5}\times \dfrac{1}{100}}{\dfrac{3}{5}\times \dfrac{2}{100}+\dfrac{2}{5}\times \dfrac{1}{100}}$

$=\dfrac{\dfrac{2}{500}}{\dfrac{6}{500}+\dfrac{2}{500}}$

$=\dfrac{2}{8}$

$=\dfrac{1}{4}$

9. Two groups are competing for the position on the board of directors of a corporation. The probabilities that the first and the second groups will win are 0.6 and 0.4 respectively. Further, if the first group wins, the probability of introducing a new product is 0.7 and the corresponding probability is 0.3 if the second group wins. Find the probability that the new product introduced was by the second group.

Ans: Let \[{{E}_{1}}\] and \[{{E}_{2}}\] be the respective events in which the first group and the second group win the competition.

\[\therefore P({{E}_{1}})=0.6\]

\[P({{E}_{2}})=0.4\]

Let A be the event of introducing a new product.

\[\Rightarrow P(A|{{E}_{1}})=\]P(introducing a new product, given that first group wins) \[=0.7\]

\[\Rightarrow P(A|{{E}_{2}})=\]P(introducing a new product, given that second group wins) \[=0.3\]

The probability that the new product is introduced by the second group, is given by \[P({{E}_{2}}|A)\]

By using Bayes theorem, we get,

$P({{E}_{2}}|A)=\dfrac{P({{E}_{2}})P(A|{{E}_{2}})}{P({{E}_{1}})P(A|{{E}_{1}})+P({{E}_{2}})P(A|{{E}_{2}})}$

$=\dfrac{0.4\times 0.3}{0.6\times 0.7+0.4\times 0.3}$

$=\dfrac{0.12}{0.42+0.12}$

$=\dfrac{0.12}{0.54}$

$=\dfrac{12}{54}$

$=\dfrac{2}{9}$

10. Suppose a girl throws a die. If she gets a 5 or 6, she tosses a coin three times and notes the number of heads. If she gets 1, 2, 3 or 4, she tosses a coin once and notes whether a head or tail is obtained. If she obtained exactly one head, what is the probability that she threw 1, 2, 3 or 4 with the die?

Ans: Let \[{{E}_{1}}\] be the event that the outcome on the die is 5 or 6 and \[{{E}_{2}}\] be the event that the outcome on the die is 1,2,3, or 4.

\[\therefore P({{E}_{1}})=\dfrac{2}{6}=\dfrac{1}{3}\]

\[P({{E}_{2}})=\dfrac{4}{6}=\dfrac{2}{3}\]

Let A be the event of getting exactly one head.

\[P({{E}_{1}})\] is same as tossing the coin 3 times, similarly \[P({{E}_{2}})\] is same as tossing the coin exactly once

\[\Rightarrow P(A|{{E}_{1}})=\]P(getting exactly one head, given the coin is tossed 3 times) \[=\dfrac{3}{8}\](TTH,THT,HTT)

\[\Rightarrow P(A|{{E}_{2}})=\]P(getting exactly one head, given the coin is tossed only once) \[=\dfrac{1}{2}\]

The probability that the girl threw 1,2,3, or 4 with die, if she obtained exactly one head, is given by \[P({{E}_{2}}|A)\]

By using Bayes theorem, we get,

$P({{E}_{2}}|A)=\dfrac{P({{E}_{2}})P(A|{{E}_{2}})}{P({{E}_{1}})P(A|{{E}_{1}})+P({{E}_{2}})P(A|{{E}_{2}})}$

$=\dfrac{\dfrac{2}{3}\times \dfrac{1}{2}}{\dfrac{1}{3}\times \dfrac{3}{8}+\dfrac{2}{3}\times \dfrac{1}{2}}$ 

$=\dfrac{\dfrac{1}{3}}{\dfrac{1}{3}\left( \dfrac{3}{8}+1 \right)}$

$=\dfrac{1}{11}\div 8$

$=\dfrac{8}{11}$

11. A manufacture has three machine operators A, B and C. The first operator A produces 1% defective items, whereas the other two operators B and C produce 5% and 7% defective items respectively. A is on the job for 50% of the time, B is on the job for 30%of the time and C is on the job for 20% of the time. A defective item is produced, what is the probability that was produced by A?

Ans: Let \[{{E}_{1}}\], \[{{E}_{2}}\] and \[{{E}_{3}}\]be the events that the time consumed by machine operator A, B and C on the job.

\[\therefore P({{E}_{1}})=50%=\dfrac{50}{100}=\dfrac{1}{2}\]

\[P({{E}_{2}})=30%=\dfrac{30}{100}=\dfrac{3}{10}\]

\[P({{E}_{3}})=20%=\dfrac{20}{100}=\dfrac{1}{5}\]

Let A be the event of producing defective items.

\[\Rightarrow P(A|{{E}_{1}})=\]P(item is defective, given that it is produced by A) \[=1%=\dfrac{1}{100}\]

\[\Rightarrow P(A|{{E}_{2}})=\]P(item is defective, given that it is produced by B) \[=5%=\dfrac{5}{100}\]

\[\Rightarrow P(A|{{E}_{3}})=\]P(item is defective, given that it is produced by C) \[=7%=\dfrac{7}{100}\]

The probability that the defective item was produced by A is given by, is given by \[P({{E}_{1}}|A)\]

By using Bayes theorem, we get,

$P({{E}_{1}}|A)=\dfrac{P({{E}_{1}})P(A|{{E}_{1}})}{P({{E}_{1}})P(A|{{E}_{1}})+P({{E}_{2}})P(A|{{E}_{2}})+P({{E}_{3}})P(A|{{E}_{3}})}$

$=\dfrac{\dfrac{1}{2}\times \dfrac{1}{100}}{\dfrac{1}{2}\times \dfrac{1}{100}+\dfrac{3}{10}\times \dfrac{5}{100}+\dfrac{1}{5}\times \dfrac{7}{100}}$

$=\dfrac{\dfrac{1}{100}\times \dfrac{1}{2}}{\dfrac{1}{100}\left( \dfrac{1}{2}+\dfrac{3}{2}+\dfrac{7}{2} \right)}$

$=\dfrac{1}{2}\div \dfrac{17}{5}$

$=\dfrac{5}{34}$

12. A card from a pack of 52 cards is lost. From the remaining cards of the pack, two cards are drawn and are found to be both diamonds. Find the probability of the lost card being a diamond.

Ans: Let \[{{E}_{1}}\] and \[{{E}_{2}}\] be the respective events of choosing a diamond card and a card which is not diamond.

Let A be the event that denote the lost card

\[\therefore P({{E}_{1}})=\dfrac{13}{52}=\dfrac{1}{4}\]

\[P({{E}_{2}})=\dfrac{39}{52}=\dfrac{3}{4}\]

When the card lost is a diamond card, then there are 12 diamond cards out of 51 cards.

From 12 diamond cards 2 cards can be drawn in \[{}^{12}{{C}_{2}}\] ways.

Therefore the probability of getting two diamond cards, when one diamond card is lost, is given by \[P(A|{{E}_{1}})\].

\[P(A|{{E}_{1}})=\dfrac{{}^{12}{{C}_{2}}}{{}^{51}{{C}_{2}}}=\dfrac{12!}{2!\times 10!}\times \dfrac{2!\times 49!}{5!}=\dfrac{22}{425}\]

When the card lost is not a diamond card, then there are 13 diamond cards out of 51 cards.

From 13 diamond cards 2 cards can be drawn in \[{}^{13}{{C}_{2}}\] ways.

Therefore the probability of getting two diamond cards, when the card lost is not a diamond, is given by \[P(A|{{E}_{2}})\].

\[P(A|{{E}_{2}})=\dfrac{{}^{13}{{C}_{2}}}{{}^{51}{{C}_{2}}}=\dfrac{13!}{2!\times 11!}\times \dfrac{2!\times 49!}{5!}=\dfrac{26}{425}\]

The probability that the lost card is diamond is given by \[P({{E}_{1}}|A)\].

$P({{E}_{1}}|A)=\dfrac{P({{E}_{1}})P(A|{{E}_{1}})}{P({{E}_{1}})P(A|{{E}_{1}})+P({{E}_{2}})P(A|{{E}_{2}})}$

$=\dfrac{\dfrac{1}{4}\times \dfrac{22}{425}}{\dfrac{1}{4}\times \dfrac{22}{425}+\dfrac{3}{4}\times \dfrac{26}{425}}$

$=\dfrac{11}{2}\div 25$

$=\dfrac{11}{50}$

13. Probability that A speaks truth is \[\dfrac{4}{5}\] . A coin is tossed. A reports that a head appears. The probability that actually there was head is

A) \[\dfrac{4}{5}\]

B) \[\dfrac{1}{2}\]

C) \[\dfrac{1}{5}\]

D) \[\dfrac{2}{5}\]

Ans: Let \[{{E}_{1}}\] and \[{{E}_{2}}\] be the events such that

\[{{E}_{1}}\] : A speaks truth 

\[{{E}_{2}}\] : A speaks false 

Let X be the event that a head appears.

\[P({{E}_{1}})=\dfrac{4}{5}\] 

\[\therefore P({{E}_{2}})=1-P({{E}_{1}})=1-\dfrac{4}{5}=\dfrac{1}{5}\] 

If a coin is tossed, then it may result in either head(H) or tail(T).

The probability of getting a head is \[\dfrac{1}{2}\] whether A speaks truth or not.

\[\therefore P(X|{{E}_{1}})=P(X|{{E}_{2}})=\dfrac{1}{2}\]

The probability that there actually a head is given by \[P({{E}_{1}}|X)\].

$P({{E}_{1}}|X)=\dfrac{P({{E}_{1}})P(X|{{E}_{1}})}{P({{E}_{1}})P(X|{{E}_{1}})+P({{E}_{2}})P(X|{{E}_{2}})}$

$=\dfrac{\dfrac{4}{5}\times \dfrac{1}{2}}{\dfrac{4}{5}\times \dfrac{1}{2}+\dfrac{1}{5}\times \dfrac{1}{2}}$ 

$=\dfrac{\dfrac{4}{5}\times \dfrac{1}{2}}{\dfrac{1}{2}\left( \dfrac{4}{5}+\dfrac{1}{5} \right)}$

$=\dfrac{4}{5}\div 1$

$=\dfrac{4}{5}$

14. If A and B are two events such that \[A\subset B\] and \[P(B)\ne 0\], then which of the following is correct?

A) \[P(A|B)=\dfrac{P(B)}{P(A)}\]

B) \[P(A|B)<P(A)\]

C) \[P(A|B)\ge P(A)\]

D) None of these

Ans: If \[A\subset B\] then \[A\cap B=A\] ,

 \[\Rightarrow A\cap B=P(A)\]

Also, \[P(A)<P(B)\]

Consider \[P(A|B)=\dfrac{P(A\cap B)}{P(B)}=\dfrac{P(A)}{P(B)}\ne \dfrac{P(B)}{P(A)}\]     - (Eq 1)

Consider \[P(A|B)=\dfrac{P(A\cap B)}{P(B)}=\dfrac{P(A)}{P(B)}\]              - (Eq 2)

It is known that, \[P(B)\le 1.\]

$\Rightarrow \dfrac{1}{P(B)}\ge$

$\Rightarrow \dfrac{P(A)}{P(B)}\ge P(A)$

From (2), we obtain

\[\Rightarrow P(A|B)\ge P(A)\]          - (Eq 3)

\[\therefore P(A|B)\] is not less than \[P(A)\].

Thus, from (3), it can be concluded that the relation given in alternate C is correct.


Conclusion

NCERT Solutions for Maths Exercise 13.3 Class 12 Chapter 13 - Probability by Vedantu help you understand Bayes' theorem clearly. Focus on how to use this theorem to solve tricky probability questions. The step-by-step solutions make learning easier. Practicing these problems will boost your understanding and help you do better in exams.


Class 12 Maths Chapter 13: Exercises Breakdown

S.No.

Chapter 13 - Probability Exercises in PDF Format

1

Class 12 Maths Chapter 13 Exercise 13.1 - 17 Questions & Solutions (4 Short Answers, 13 Long Answers)

2

Class 12 Maths Chapter 13 Exercise 13.2 - 18 Questions & Solutions

3

Class 12 Maths Chapter 13 Miscellaneous Exercise - 17 Questions & Solutions



CBSE Class 12 Maths Chapter 13 Other Study Materials



NCERT Solutions for Class 12 Maths | Chapter-wise List

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FAQs on CBSE Class 12 Maths Chapter 13 Probability – NCERT Solutions Exercise 13.3 [2025-26]

1. How are NCERT Solutions for Class 12 Maths Chapter 13 Exercise 13.3 helpful in understanding conditional probability?

These NCERT Solutions provide stepwise explanations on applying conditional probability and related theorems to problems as per the latest CBSE syllabus. By focusing on the method of defining events, clearly outlining what is 'given', and using formulas like P(A|B) = P(A ∩ B)/P(B), students develop a clear and exam-ready approach to tricky probability concepts.

2. What type of step-by-step methods should I follow when solving NCERT Exercise 13.3 problems?

Begin with defining all relevant events (e.g., A and B). Then, identify any ‘given that’ condition for clarity. Apply the multiplication law or conditional probability formula as needed, always calculating denominators first in conditional questions. Box key results and clearly mention each calculated probability step.”

3. Which are the most common mistakes students make while solving conditional probability questions in Exercise 13.3?

  • Mixing up formulas for independent and dependent events
  • Forgetting to calculate the correct denominator in conditional probability
  • Not defining events properly before solving
  • Skipping calculation steps, leading to errors

Always check for ‘given that’ in the question and ensure all events and steps are well-defined in your solution.

4. How do NCERT Solutions ensure the correct application of Bayes’ Theorem in board-style problems?

These solutions guide you to identify prior and conditional probabilities explicitly, then substitute correctly into Bayes’ Theorem as required by CBSE board exam standards. All calculations follow syllabus-approved logic, helping students avoid common pitfalls and ensuring full marks in proof-based answers.

5. What is the main difference between independent events and mutually exclusive events, and how is this shown in the solutions?

  • Independent events: The outcome of one event does not impact the probability of the other. Shown by using the multiplication law P(A ∩ B) = P(A) × P(B).
  • Mutually exclusive events: Both cannot occur at the same time. Addressed by the addition law P(A ∪ B) = P(A) + P(B).

NCERT stepwise solutions clarify these with clear event definitions so you apply the correct approach in each case.

6. How can I check if a question from Exercise 13.3 is asking for conditional probability?

Look for phrases like ‘given that’ or wording indicating an event has already occurred. If the problem requires you to find the probability of one event assuming another has happened, use P(A | B). Reading the question carefully for such keywords is essential to avoid applying the wrong formula.

7. Why are stepwise NCERT solutions preferred for Class 12 board exams in probability?

Stepwise solutions help you structure your answer logically, making it easy for examiners to award marks for each stage as per CBSE guidelines. You show clear event definitions, formulas used, substitution, and final boxed answers—this increases accuracy and partial marking opportunities.

8. What key probability formulas should I revise repeatedly before the board exam?

  • P(A ∩ B) = P(A) × P(B) (for independent events)
  • P(A|B) = P(A ∩ B)/P(B) (conditional probability)
  • P(A ∪ B) = P(A) + P(B) – P(A ∩ B) (addition law)
  • Bayes’ theorem for reversing conditional probability

Creating a summary sheet with boxed formulas and practicing their usage in multiple questions from Exercise 13.3 reinforces exam readiness.

9. How do these solutions help in differentiating when to use Bayes’ Theorem versus direct conditional probability?

If you need to find the probability of a cause given an observed effect (i.e., reverse probability), Bayes’ Theorem is used. If you are calculating the probability of an event with another already occurring, use direct conditional probability. The solutions walk you through both situations in the syllabus context.

10. What strategies improve accuracy when answering probability word problems in the board exam?

  • Define all events and what is ‘given’ before you start calculations
  • Draw probability trees or diagrams for clarity if needed
  • Check if the question involves dependency or independence
  • Double-check denominators in conditional calculations
  • Practice with a variety of solved examples from the NCERT Exercise 13.3

11. In what ways are conditional probability and independence of events tested in Exercise 13.3?

Exercise 13.3 includes questions where you assess if two events are independent (their joint probability equals the product of their probabilities) and apply conditional probability when the occurrence of one event affects the probability of another. Some problems require you to use both concepts together to test conceptual understanding.

12. How do NCERT Solutions address the hardest board-style questions on probability?

The hardest questions usually combine multiple concepts, such as Bayes’ Theorem, conditional probability, and event independence. NCERT Solutions demonstrate how to break down such problems into smaller definable events, guide through all steps, and highlight where most students make mistakes, providing strategies for systematic, accurate answers.

13. What should I do if I get confused between mutually exclusive and independent events in a problem?

Remember that mutually exclusive events never occur together (P(A ∩ B) = 0), while independent events can occur together but do not influence each other’s probabilities. Re-read definitions in the NCERT textbook and review example problems that clarify these distinctions.

14. How are real-life scenarios like medical tests or coin tosses represented in probability solutions?

Real-life scenarios are translated into defined events (such as test outcomes as A/B, or heads/tails). The NCERT Solutions teach students to model these with formal probability language, apply the appropriate formulas, and interpret results as per the Class 12 Maths syllabus for probability.