All questions of Probability for JEE Exam

A six faced fair dice is thrown until 1 comes, then the probability that 1 comes in even no. of trials is (2005S)
  • a)
    5/11
  • b)
    5/6
  • c)
    6/11
  • d)
    1/6
Correct answer is option 'A'. Can you explain this answer?

Krishna Iyer answered
In single throw of a dice, probability of getting 1 is =  and prob. of not getting 1 is 
Then getting 1 in even no. of chances = getting 1 in 2nd chance or in 4th chance or in 6th chance and so on
∴ Req. Prob 

The probability of India winning a test match against west Indies is 1/2. Assuming independence from match to match the probability that in a 5 match series India’s second win occurs at third test is (1995S)
  • a)
    1/8
  • b)
    1/4
  • c)
    1/2
  • d)
    2/3
Correct answer is option 'B'. Can you explain this answer?

Kirti Datta answered
Wins at least 3 matches can be calculated using the binomial distribution.

The binomial distribution calculates the probability of getting a certain number of successes (India winning a match) in a fixed number of independent Bernoulli trials (5 matches).

To find the probability that India wins at least 3 matches in a 5-match series, we need to calculate the probabilities of winning 3, 4, and 5 matches and then sum them up.

P(India wins at least 3 matches) = P(India wins 3 matches) + P(India wins 4 matches) + P(India wins 5 matches)

P(India wins k matches) = (5Ck) * (1/2)^k * (1/2)^(5-k)

Where (5Ck) is the binomial coefficient that represents the number of ways to choose k successes from 5 trials.

Calculating each term separately:

P(India wins 3 matches) = (5C3) * (1/2)^3 * (1/2)^(5-3) = 10 * (1/8) * (1/8) = 10/64 = 5/32

P(India wins 4 matches) = (5C4) * (1/2)^4 * (1/2)^(5-4) = 5 * (1/16) * (1/2) = 5/32

P(India wins 5 matches) = (5C5) * (1/2)^5 * (1/2)^(5-5) = 1 * (1/32) * (1/1) = 1/32

Now, summing up the probabilities:

P(India wins at least 3 matches) = 5/32 + 5/32 + 1/32 = 11/32

Therefore, the probability that India wins at least 3 matches in a 5-match series against West Indies is 11/32.

 Four  fair  dice  D1, D2, D3 and  D4 ;  each  having  six  faces numbered  1,  2,  3,  4,  5  and  6  are  rolled simultaneously.
The probability that D4  shows a number appearing on one of D1, D2 and D3 is (2012)
  • a)
  • b)
  • c)
  • d)
Correct answer is option 'A'. Can you explain this answer?

Anaya Patel answered
D4 can show an umber appearing on one of D1, D2 and D3 in the following cases.
Case I : D4 shows a number which is shown by only one of D1, D2 and D3.
D4 shows a number in 6C1 ways.
One out of D1, D2 and D3 can be selected in 3C1 ways.
The selected die shows the same number as on D4 in one way and rest two dice show the different number in 5 ways each.
∴ Number of ways to happen case I = 6C1 × 3C1 × 1 × 5 × 5 = 450
Case II : D4 shows a number which is shown by only two of D1, D2 and D3.
As discussed in case I, it can happen in the following number of ways = 6C1 × 3C2 × 1 × 1 × 5 = 90
Case III : D4 shows a number which is shown by all three dice D1, D2 and D3.
Number of ways it can be done = 6C1 × 3C3 × 1 × 1 × 1 = 6
∴ Total number of favourable ways = 450 + 90 + 6 = 546
Also total ways = 6 × 6 × 6 × 6
∴ Required Probability =  

Let ω be a complex cube root of unity with ω ≠ 1. A fair die is thrown three times. If r1, r2 and r3 are the numbers obtained on the die, then the probability that ωr1 + ωr2 + ωr3 = 0 is (2010)
  • a)
  • b)
  • c)
  • d)
Correct answer is option 'C'. Can you explain this answer?

Chirag Verma answered
If ω is a complex cube root of unity then, we know that ω3m 3n+13p +2 = 0 where m, n, p are integers.
∴ r1, r2, r3 should be of the form 3m, 3n + 1 and 3p + 2 taken in any or d er. As r1 ,r2 ,r3 are the numbers obtained on die, these can take any value from 1 to 6.
∴ m can take values 1 or 2, n can take values 0 or 1p can take values 0 or 1
∴ Number of ways of selecting r1, r2, r3 
= 2C1 x 2C1 x 2C1 x 3!.
Also the total number of ways of getting r1, r2, r3 on die = 6 × 6 × 6
∴ Required probability=

Let Ec denote the complement of an event E. Let E, F, G be pairwise independent events with P(G) > 0 and P(E∩F∩G) = 0. Then P(Ec∩ Fc| G) equals   (2007 -3 marks)
  • a)
    P(Ec) + P(Fc)
  • b)
    P(Ec) – P(Fc)
  • c)
    P(Ec) – P(F)
  • d)
    P(E) – P(Fc)
Correct answer is option 'C'. Can you explain this answer?

Siddharth Rane answered
P( Ec ∩ Fc /G) = P( E ∪ F )c /G) 1 - P(E∪ F /G)
= 1 - P(E / G) - P(F/ G) + P(E∩ F / G)
= 1 - P(E ) -P(F)+O
(∵ E, F, G are pairwise independent and
P(E ∩F ∩G)=0
⇒ P(E ).P(F )=0 as P(G) >0 ) = P(Ec )- P(F)

One hundred identical coins, each with probability, p, of showing up heads are tossed once. If 0 < p < 1 and the probabilitity of heads showing on 50 coins is equal to that of heads showing on 51 coins, then the value of p is (1988 - 2 Marks)
  • a)
    1/2
  • b)
    49/101
  • c)
    50/101
  • d)
    51/101.
Correct answer is option 'D'. Can you explain this answer?

Prob. of one coin showing head = p
∴ Prob of one coin showing tail = 1– p ATQ coin is tossed 100 times and prob. of 50 coins showing head = prob of 51 coins showing head.
Using binomial prob. distribution
P (X = r)= nCr prqn-r ,
we get,    
⇒ 51 – 51 p = 50 p
⇒ 101 p = 51 ⇒ 

If M and N are any two events, the probability that exactly one of them occurs is (1984 - 3 Marks)
  • a)
    P(M ) + P( N ) - 2P (M ∩ N)
  • b)
    (P(M ) + P(N) - P (M ∩ N)
  • c)
    P(Mc) + P(Nc) - 2P(Mc ∩ Nc)
  • d)
    P(M ∩ Nc) + P(Mc ∩ N)
Correct answer is option 'A,C,D'. Can you explain this answer?

Jatin Iyer answered
Given that M and N are any two events. To check the probability that exactly one of them occurs.
We check all the options one by one.
(a) P (M) + P (N) – 2 P (M ∩ N) = [P (M) + P (N) – P (M ∩ N)] – P( M ∩ N)
= P (M ∪ N) – P (M ∩ N)
⇒ Prob. that exactly one of M and N occurs.
(b) P (M) + P (N) – P (M ∩ N) = P (M ∪ N)
⇒ Prob. that at least one of M and N occurs.
(c) P (Mc) + P (Nc) – 2 P(Mc ∩ Nc) = 1– P (M) + 1 – P (N) – 2P (M ∪ N)c
= 2 – P (M)  – P (N) – 2 [1 – P (M ∪ N)] 
= 2 – P(M) – P (N) – 2 + 2 P (M ∪ N)
= P (M ∪ N) +  P (M ∪ N) – P(M) – P (N)
= P (M ∪ N) – P (M ∩ N)
⇒ Prob. that exactly one of M and N occurs.
(d) P (M ∩ Nc) + P (M∩ N)
⇒ Prob that M occurs but not N or prob that M does not occur but N occurs.
⇒ Prob. that exactly one of M and N occurs.
Thus we can conclude that (a), (c) and (d) are the correct options.

Let A , B , C be three mutually independent events.Consider the two statements S1 and S2 S1 : A and B ∪ C are independent S2 : A an d B ∩ C are independent Then,(1994)
  • a)
    Both S1 an d S2 are true
  • b)
    Only S1 is tr ue
  • c)
    Only S2 is true
  • d)
    Neither S1 nor S2 is true
Correct answer is option 'A'. Can you explain this answer?

Uday Ghoshal answered
P [A ∩ (B ∪ C)] = P [(A ∩ B) ∪ (A ∩ C)] = P (A ∩ B) + P (A ∩ C) – P (A ∩ B ∩ C)
= P (A)  P (B) + P (A)  P (C) – P (A)  P (B) P(C)
= P (A) [P (B) + P (C) – P (B ∩ C)] = P (A) P (B ∪ C)
∴ S1 is true.
P (A ∩ (B ∩ C)) = P (A) P(B)  P (C) = P (A) P (B ∩ C)
∴ S2 is also true.

Three numbers are chosen at random without replacement from {1,2,3,..8}. The probability that their minimum is 3, given that their maximum is 6, is : [2012]
  • a)
  • b)
  • c)
  • d)
Correct answer is option 'B'. Can you explain this answer?

Arjun Sen answered
Given sample space = {1,2,3,.....,8}
Let Event A : Maximum of three numbers is 6.
B : Minimum of three numbers is 3.
This is the case of conditional probability
We have to find P (minimum) is 3 when it is given that P (maximum) is 6.

Five  horses  are  in a race.  Mr. A  selects two of  the horses  at random and  bets on them. The  probability that Mr. A selected the winning horse is [2003]
  • a)
  • b)
  • c)
  • d)
Correct answer is option 'A'. Can you explain this answer?

Nisha Sen answered
Let 5 horses are H1, H2, H3, H4 and H5. Selected pair of horses will be one of the 10 pairs (i.e.; 5C2 ): H1 H2, H1 H3, H1 H4, H1 H5, H2H3, H2 H4, H2 H5, H3 H4, H3 H5
and H4 H5.
Any horse can win the race in 4 ways.
For example : Horses H2 win the race in 4 ways H1 H2, H2H3, H2H4 and H2H5.
Hence required probability =

A random variable X has the probability distribution:  For the events E = {X is a prime number } and F = {X< 4}, the P( E ∪ F) is [2004]
  • a)
    0.50
  • b)
    0.77
  • c)
    0.35
  • d)
    0.87
Correct answer is option 'B'. Can you explain this answer?

P(E) = P ( 2 or 3 or 5 or 7) = 0.23 + 0.12 + 0.20 + 0.07 = 0.62
P( F ) =P(1 or2 or3) = 0.15 + 0.23 + 0.12 = 0.50 P( E ∩ F )=P(2 or 3) = 0.23 + 0.12 = 0.35
∴ P(EUF ) = P(E) + P(F ) - P(E∩ F )
= 0.62 + 0.50 - 0.35 = 0.77

For any two events A and B in a sample space (1991 -  2 Marks)
  • a)
    is always true
  • b)
    does not hold
  • c)
    , if A and B are independent
  • d)
    , if A and B are disjoint.
Correct answer is option 'A,C'. Can you explain this answer?

Isha Choudhury answered
For any two events A and B
(a)
Now we know P (A ∪ B) ≤ 1 P (A) + P (B) – P (A ∩ B) ≤ 1
⇒ P (A ∩ B) > P (A) + P (B) – 1
         ∴ (a) is correct statement.
(b)
From venn diagram we can clearly conclude that
∴ (b) is incorrect statement.
 
(c) P (A ∪ B) =P (A) + P (B) – P (A ∩ B)
[ ∵ A & B are independent events]
         ∴ (c) is the correct statement.
(d) For disjoint events P (A ∪ B ) = P (A) + P (B)
∴ (d) is the incorrect statement.

Two events A an d B ha ve pr obabilities 0.25 an d 0.50 respectively. The probability that both A and B occur simultaneously is 0.14. Then the probability that neither A nor B occurs is (1980)
  • a)
    0.39
  • b)
    0.25
  • c)
    0.11
  • d)
    none of these
Correct answer is option 'A'. Can you explain this answer?

Yeah its 0.39
solving with van diagram we get as...
Total prob=1
p(a)=.25
p(b)=.50
and p(a intersection b)=.14
thus by van diagram...
1-(0.25+0.50-0.14) bcoz 0.14 was included 2 times... once in 0.25 and the other time in0.50.
ans is 0.39

In dia plays two matches each with West In dies and Australia. In any match the probabilities of India getting, points 0, 1 and 2 are 0.45, 0.05 and 0.50 respectively.Assuming that the outcomes are independent, the probability of India getting at least 7 points is (1992 -  2 Marks)
  • a)
    0.8750
  • b)
    0.0875
  • c)
    0.0625
  • d)
    0.0250
Correct answer is option 'B'. Can you explain this answer?

Pallabi Basak answered
P (at least 7 pts) = P (7pts) + P (8 pts) [∵ At most 8 pts can be scored.]
Now 7 pts can be scored by scoring 2 pts in 3 matches and 1 pt. in one match, similarly 8 pts can be scored by scoring 2 pts in each of the 4 matches.
∴ Req. prob. = 4C1 × [P (2 pts)]3 P (1pt) + [P (2 pts)]4 = 4 (0.5)3 × 0.05 + (0.50)4  = 0.0250 + 0.0625 = 0.0875

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