CBSE · NCERT · Class 11 Maths · Chapter 14

NCERT Solutions: Class 11 Maths Chapter 14 - Probability

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Chapter-wise NCERT intext questions and exercise answers for Probability, grounded in the official textbook.

Questions are taken verbatim from the NCERT textbook; answers were grounded against the chapter's content during generation. Items needing review are marked.
Sections in this chapter
Exercise 14.1 7Exercise 14.2 10Miscellaneous Exercise on Chapter 14 10
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1Exercise 14.17 questions
Q.1A die is rolled. Let E be the event “die shows 4” and F be the event “die shows even number”. Are E and F mutually exclusive?v
Solution

Here $E=\{4\}$ and $F=\{2,4,6\}$. Since $E\cap F=\{4\}\ne\varnothing$, the events can occur together and are not mutually exclusive.

Answer:

No, E and F are not mutually exclusive.

Q.2A die is thrown. Describe the following events: (i) A: a number less than 7 (ii) B: a number greater than 7 (iii) C: a multiple of 3 (iv) D: a number less than 4 (v) E: an even number greater than 4 (vi) F: a number not less than 3 Also find A ∪ B, A ∩ B, B ∪ C, E ∩ F, D ∩ E, A – C, D – E, E ∩ F′, F′v
Solution

The sample space is $S=\{1,2,3,4,5,6\}$. Write each event as a subset of $S$ and then apply union, intersection, difference and complement operations directly.

Answer:

$A=\{1,2,3,4,5,6\}$, $B=\varnothing$, $C=\{3,6\}$, $D=\{1,2,3\}$, $E=\{6\}$, $F=\{3,4,5,6\}$; $A\cup B=A$, $A\cap B=\varnothing$, $B\cup C=\{3,6\}$, $E\cap F=\{6\}$, $D\cap E=\varnothing$, $A-C=\{1,2,4,5\}$, $D-E=\{1,2,3\}$, $E\cap F'=\varnothing$, $F'=\{1,2\}$.

Q.3An experiment involves rolling a pair of dice and recording the numbers that come up. Describe the following events: A: the sum is greater than 8, B: 2 occurs on either die C: the sum is at least 7 and a multiple of 3. Which pairs of these events are mutually exclusive?v
Solution

List ordered pairs from two dice. Event $C$ consists of sums 9 and 12. Since $C\subset A$, $A$ and $C$ are not mutually exclusive. No outcome in $B$ has sum greater than 8 or sum 9 or 12, so $A\cap B=\varnothing$ and $B\cap C=\varnothing$.

Answer:

$A=\{(3,6),(4,5),(4,6),(5,4),(5,5),(5,6),(6,3),(6,4),(6,5),(6,6)\}$; $B=\{(2,1),(2,2),(2,3),(2,4),(2,5),(2,6),(1,2),(3,2),(4,2),(5,2),(6,2)\}$; $C=\{(3,6),(4,5),(5,4),(6,3),(6,6)\}$. The mutually exclusive pairs are $(A,B)$ and $(B,C)$.

Q.4Three coins are tossed once. Let A denote the event ‘three heads show”, B denote the event “two heads and one tail show”, C denote the event” three tails show and D denote the event ‘a head shows on the first coin”. Which events are (i) mutually exclusive? (ii) simple? (iii) Compound?v
Solution

Here $A=\{HHH\}$, $B=\{HHT,HTH,THH\}$, $C=\{TTT\}$ and $D=\{HHH,HHT,HTH,HTT\}$. Simple events contain one sample point; compound events contain more than one. Mutually exclusive pairs have empty intersection.

Answer:

(i) Mutually exclusive pairs: $A$ and $B$, $A$ and $C$, $B$ and $C$, $C$ and $D$. (ii) Simple events: $A$ and $C$. (iii) Compound events: $B$ and $D$.

Q.5Three coins are tossed. Describe (i) Two events which are mutually exclusive. (ii) Three events which are mutually exclusive and exhaustive. (iii) Two events, which are not mutually exclusive. (iv) Two events which are mutually exclusive but not exhaustive. (v) Three events which are mutually exclusive but not exhaustive.v
Solution

For three coin tosses, $S=\{HHH,HHT,HTH,THH,HTT,THT,TTH,TTT\}$. The listed examples are checked by intersections: mutually exclusive events have no common outcomes, and exhaustive events have union equal to $S$.

Answer:

One possible answer: (i) $A=$ no head and $B=$ exactly one head. (ii) no head, exactly one head, at least two heads. (iii) at least one head and at least one tail. (iv) no head and three heads. (v) no head, exactly one head, exactly two heads.

Q.6Two dice are thrown. The events A, B and C are as follows: A: getting an even number on the first die. B: getting an odd number on the first die. C: getting the sum of the numbers on the dice ≤ 5. Describe the events (i) A′ (ii) not B (iii) A or B (iv) A and B (v) A but not C (vi) B or C (vii) B and C (viii) A ∩ B′ ∩ C′v
Solution

Since $A$ and $B$ are complementary events, $A'=B$ and $B'=A$. Event $C'$ means the sum is greater than 5. Substitute these descriptions into each requested expression.

Answer:

(i) odd number on the first die; (ii) even number on the first die; (iii) all outcomes; (iv) impossible event; (v) even first die and sum greater than 5; (vi) odd first die or sum at most 5; (vii) odd first die and sum at most 5; (viii) even first die and sum greater than 5.

Q.7Refer to question 6 above, state true or false: (give reason for your answer) (i) A and B are mutually exclusive (ii) A and B are mutually exclusive and exhaustive (iii) A = B′ (iv) A and C are mutually exclusive (v) A and B′ are mutually exclusive. (vi) A′, B′, C are mutually exclusive and exhaustive.v
Solution

$A$ is even first die and $B$ is odd first die, so they are complements. Thus (i), (ii) and (iii) are true. For (iv), $A\cap C$ is not empty, for example $(2,1)$. For (v), $B'=A$, so $A\cap B'=A\ne\varnothing$. For (vi), $A'=B$ and $B'=A$ overlap with $C$, so the three events are not mutually exclusive, though their union covers the sample space.

Answer:

(i) True; (ii) True; (iii) True; (iv) False; (v) False; (vi) False.

2Exercise 14.210 questions
Q.1Which of the following can not be valid assignment of probabilities for outcomes of sample Space S = {ω1,ω2,ω3,ω4,ω5,ω6,ω7}v
Solution

A valid probability assignment must have all probabilities non-negative and total probability 1. In (c), the sum is $0.1+0.2+\cdots+0.7=2.8>1$. In (d), some probabilities are negative. In (e), the sum is $\dfrac{1+2+3+4+5+6+15}{14}=\dfrac{36}{14}>1$. Assignments (a) and (b) are valid.

Answer:

Assignments (c), (d) and (e) cannot be valid.

Q.2A coin is tossed twice, what is the probability that atleast one tail occurs?v
Solution

The sample space is $\{HH,HT,TH,TT\}$. Outcomes with at least one tail are $HT,TH,TT$, so the probability is $\dfrac{3}{4}$.

Answer:

$\dfrac34$.

Q.3A die is thrown, find the probability of following events: (i) A prime number will appear, (ii) A number greater than or equal to 3 will appear, (iii) A number less than or equal to one will appear, (iv) A number more than 6 will appear, (v) A number less than 6 will appear.v
Solution

For a die, $S=\{1,2,3,4,5,6\}$. Prime outcomes are $\{2,3,5\}$. Outcomes $\ge3$ are $\{3,4,5,6\}$. Outcomes $\le1$ are $\{1\}$. No outcome is more than 6. Outcomes less than 6 are $\{1,2,3,4,5\}$.

Answer:

(i) $\dfrac12$; (ii) $\dfrac23$; (iii) $\dfrac16$; (iv) $0$; (v) $\dfrac56$.

Q.4A card is selected from a pack of 52 cards. (a) How many points are there in the sample space? (b) Calculate the probability that the card is an ace of spades. (c) Calculate the probability that the card is (i) an ace (ii) black card.v
Solution

There are 52 cards. The ace of spades is one card, so probability $=1/52$. There are 4 aces, so probability of an ace $=4/52=1/13$. There are 26 black cards, so probability $=26/52=1/2$.

Answer:

(a) $52$; (b) $\dfrac1{52}$; (c)(i) $\dfrac1{13}$; (c)(ii) $\dfrac12$.

Q.5A fair coin with 1 marked on one face and 6 on the other and a fair die are both tossed. find the probability that the sum of numbers that turn up is (i) 3 (ii) 12v
Solution

There are $2\times6=12$ equally likely outcomes. Sum 3 occurs only when the coin shows 1 and the die shows 2. Sum 12 occurs only when the coin shows 6 and the die shows 6. Each probability is $\dfrac1{12}$.

Answer:

(i) $\dfrac1{12}$; (ii) $\dfrac1{12}$.

Q.6There are four men and six women on the city council. If one council member is selected for a committee at random, how likely is it that it is a woman?v
Solution

There are $4+6=10$ council members, of whom 6 are women. Probability $=\dfrac6{10}=\dfrac35$.

Answer:

$\dfrac35$.

Q.7A fair coin is tossed four times, and a person win Re 1 for each head and lose Rs 1.50 for each tail that turns up. From the sample space calculate how many different amounts of money you can have after four tosses and the probability of having each of these amounts.v
Solution

If $h$ heads occur in four tosses, tails are $4-h$ and the amount is $h(1)-(4-h)(1.50)=2.5h-6$. For $h=0,1,2,3,4$, this gives Rs $-6,-3.50,-1,1.50,4$. The probabilities are $\binom4h/16$, i.e. $1/16,4/16,6/16,4/16,1/16$.

Answer:

The possible amounts are Rs $-6,-3.50,-1,1.50,4$ with probabilities $\dfrac1{16},\dfrac4{16},\dfrac6{16},\dfrac4{16},\dfrac1{16}$ respectively.

Q.8Three coins are tossed once. Find the probability of getting (i) 3 heads (ii) 2 heads (iii) atleast 2 heads (iv) atmost 2 heads (v) no head (vi) 3 tails (vii) exactly two tails (viii) no tail (ix) atmost two tailsv
Solution

There are 8 equally likely outcomes. Counts are: 3 heads: 1; exactly 2 heads: 3; at least 2 heads: 4; at most 2 heads: 7; no head: 1; 3 tails: 1; exactly 2 tails: 3; no tail: 1; at most 2 tails: 7. Divide each count by 8.

Answer:

(i) $\dfrac18$; (ii) $\dfrac38$; (iii) $\dfrac12$; (iv) $\dfrac78$; (v) $\dfrac18$; (vi) $\dfrac18$; (vii) $\dfrac38$; (viii) $\dfrac18$; (ix) $\dfrac78$.

Q.9If $\dfrac12$ is the probability of an event, what is the probability of the event ‘not A’.v
Solution

For any event $A$, $P(A')=1-P(A)$. Given $P(A)=\dfrac12$, $P(A')=1-\dfrac12=\dfrac12$.

Answer:

$\dfrac12$.

Q.10A letter is chosen at random from the word ‘ASSASSINATION’. Find the probability that letter is (i) a vowel (ii) a consonantv
Solution

The word ASSASSINATION has 13 letters. Vowels are A, A, I, A, I, O, i.e. 6 letters. Consonants are the remaining 7 letters. Therefore the probabilities are $6/13$ and $7/13$.

Answer:

(i) $\dfrac6{13}$; (ii) $\dfrac7{13}$.

3Miscellaneous Exercise on Chapter 1410 questions
Q.1A box contains 10 red marbles, 20 blue marbles and 30 green marbles. 5 marbles are drawn from the box, what is the probability that (i) all will be blue? (ii) atleast one will be green?v
Solution

There are 60 marbles, so total ways to draw 5 are $\binom{60}{5}$. (i) All blue can be chosen in $\binom{20}{5}$ ways. (ii) At least one green is the complement of drawing all 5 from the 30 non-green marbles, so probability is $1-\dfrac{\binom{30}{5}}{\binom{60}{5}}$.

Answer:

(i) $\dfrac{\binom{20}{5}}{\binom{60}{5}}$; (ii) $1-\dfrac{\binom{30}{5}}{\binom{60}{5}}$.

Q.24 cards are drawn from a well – shuffled deck of 52 cards. What is the probability of obtaining 3 diamonds and one spade?v
Solution

Total ways to draw 4 cards are $\binom{52}{4}$. Choose 3 diamonds from 13 and 1 spade from 13, giving $\binom{13}{3}\binom{13}{1}$ favourable draws. Hence the probability is $\dfrac{\binom{13}{3}\binom{13}{1}}{\binom{52}{4}}$.

Answer:

$\dfrac{\binom{13}{3}\binom{13}{1}}{\binom{52}{4}}$.

Q.3A die has two faces each with number ‘1’, three faces each with number ‘2’ and one face with number ‘3’. If die is rolled once, determine (i) P(2) (ii) P(1 or 3) (iii) P(not 3)v
Solution

The die has 6 faces: two labelled 1, three labelled 2, and one labelled 3. Thus $P(2)=3/6=1/2$, $P(1\text{ or }3)=(2+1)/6=1/2$, and $P(\text{not }3)=5/6$.

Answer:

(i) $\dfrac12$; (ii) $\dfrac12$; (iii) $\dfrac56$.

Q.4In a certain lottery 10,000 tickets are sold and ten equal prizes are awarded. What is the probability of not getting a prize if you buy (a) one ticket (b) two tickets (c) 10 tickets.v
Solution

There are 9990 non-prize tickets among 10000. For one ticket, probability of no prize is $9990/10000=999/1000$. For two tickets, both must come from the 9990 non-prize tickets, giving $\dfrac{\binom{9990}{2}}{\binom{10000}{2}}$. For 10 tickets, probability $=\dfrac{\binom{9990}{10}}{\binom{10000}{10}}$.

Answer:

(a) $\dfrac{999}{1000}$; (b) $\dfrac{\binom{9990}{2}}{\binom{10000}{2}}$; (c) $\dfrac{\binom{9990}{10}}{\binom{10000}{10}}$.

Q.5Out of 100 students, two sections of 40 and 60 are formed. If you and your friend are among the 100 students, what is the probability that (a) you both enter the same section? (b) you both enter the different sections?v
Solution

If you are in the section of 40, your friend is in the same section with probability $39/99$; this case has probability $40/100$. If you are in the section of 60, your friend is in the same section with probability $59/99$; this case has probability $60/100$. Thus probability of same section is $\dfrac{40}{100}\dfrac{39}{99}+\dfrac{60}{100}\dfrac{59}{99}=\dfrac{17}{33}$. Probability of different sections is $1-\dfrac{17}{33}=\dfrac{16}{33}$.

Answer:

(a) $\dfrac{17}{33}$; (b) $\dfrac{16}{33}$.

Q.6Three letters are dictated to three persons and an envelope is addressed to each of them, the letters are inserted into the envelopes at random so that each envelope contains exactly one letter. Find the probability that at least one letter is in its proper envelope.v
Solution

There are $3!=6$ ways to place the letters. The number of derangements of 3 letters is 2, so the probability that no letter is in the proper envelope is $2/6=1/3$. Therefore the probability that at least one letter is in its proper envelope is $1-1/3=2/3$.

Answer:

$\dfrac23$.

Q.7A and B are two events such that P(A) = 0.54, P(B) = 0.69 and P(A ∩ B) = 0.35. Find (i) P(A ∪ B) (ii) P(A´ ∩ B´) (iii) P(A ∩ B´) (iv) P(B ∩ A´)v
Solution

$P(A\cup B)=P(A)+P(B)-P(A\cap B)=0.54+0.69-0.35=0.88$. Then $P(A'\cap B')=1-P(A\cup B)=0.12$. Also $P(A\cap B')=P(A)-P(A\cap B)=0.19$ and $P(B\cap A')=P(B)-P(A\cap B)=0.34$.

Answer:

(i) $0.88$; (ii) $0.12$; (iii) $0.19$; (iv) $0.34$.

Q.8From the employees of a company, 5 persons are selected to represent them in the managing committee of the company. Particulars of five persons are as follows: S. No. Name Sex Age in years 1. Harish M 30 2. Rohan M 33 3. Sheetal F 46 4. Alis F 28 5. Salim M 41 A person is selected at random from this group to act as a spokesperson. What is the probability that the spokesperson will be either male or over 35 years?v
Solution

Males are Harish, Rohan and Salim. Persons over 35 are Sheetal and Salim. The union contains Harish, Rohan, Salim and Sheetal, i.e. 4 persons out of 5. Probability $=4/5$.

Answer:

$\dfrac45$.

Q.9If 4-digit numbers greater than 5,000 are randomly formed from the digits 0, 1, 3, 5, and 7, what is the probability of forming a number divisible by 5 when, (i) the digits are repeated? (ii) the repetition of digits is not allowed?v
Solution

(i) With repetition, the first digit is 5 or 7, the middle two digits have 5 choices each, and the last digit must be 0 or 5. Favourable numbers $=2\cdot5\cdot5\cdot2=100$, total $=2\cdot5^3=250$, so probability $=2/5$. (ii) Without repetition, total numbers $=2\cdot{}^4P_3=48$. Favourable cases: if first digit is 5, last must be 0 and the middle two have $3P2=6$ choices; if first digit is 7, last can be 0 or 5, giving $6+6=12$ choices. Favourable $=18$, so probability $=18/48=3/8$.

Answer:

(i) $\dfrac25$; (ii) $\dfrac38$.

Q.10The number lock of a suitcase has 4 wheels, each labelled with ten digits i.e., from 0 to 9. The lock opens with a sequence of four digits with no repeats. What is the probability of a person getting the right sequence to open the suitcase?v
Solution

The number of possible four-digit sequences with no repeats from 10 digits is ${}^{10}P_4=10\cdot9\cdot8\cdot7=5040$. Only one sequence opens the lock, so the probability is $\dfrac1{5040}$.

Answer:

$\dfrac1{5040}$.