Free Probability 02 Practice Test - 12th Grade - Commerce
Question 1
A random variable X has the following probability distribution
XP(X=x)XP(X=x)0λ511λ13λ613λ25λ715λ37λ817λ49λ
then, λ is equal to
SOLUTION
Solution : A
∑8x=0P(X)=1⇒P(0)+P(1)+P(2)+P(3)+P(4)+P(5)+P(6)+P(7)+P(8)=1=λ+3λ+5λ+7λ+9λ+11λ+13λ+15λ+17λ=1⇒92(λ+17λ)=1⇒81λ=1∴λ=181
Question 2
A man is known to speak truth is 75% cases. If he throws an unbiased die and tells his friend that it is a six, then the probability that it is actually a six, is
SOLUTION
Solution : D
Let E denote the event that a six occurs and A be the event theat the man reports that it is a six. We have P(E)=16,P(E′)=56,P(AE)=34 and P(AE′)=14. By Baye’s theorem
P(EA)=P(E).P(AE)P(E).P(AE)+P(E′).P(AE′)=(16)(34)(16)(34)+(56)(14)=38
Question 3
Seven coupons are selected at random one at a time with replacement from 15 coupons numbered 1 to 15. The probability that the largest number appearing on a selected coupon is 9, is
SOLUTION
Solution : D
Each coupon can be selected in 15 ways. The total number of ways of choosing 7 copouns is 157. If largest number is 9, then the selected numbers have to be from 1 to 9 excluding those consisting of only 1 to 8.
Probability desired is 97−87157
=(35)7−(815)7
Question 4
Team A plays with 5 other teams exactly once. Assuming that for each match the probabilities of a win, draw and loss are equal, then
SOLUTION
Solution : B
Probability of equal number of W and L is the probability of following
(0)W,(0)L +(1)W,(1)L+(2)W, (2)L
=(13)5+5C1.4C1(13)5+5C2.3C2(13)5=1781
Question 5
The probabilities of three mutually exclusive events A, B, C are given by 23,14, and 16 respectively. This statement
SOLUTION
Solution : B
Since the events A,B,C are mutually exclusive, we have
P(A⋃B⋃C)=23+14+16=1312>0
Which is not possible , hence the statement is false.
Question 6
Two events A and B have the probabilities 0.25 and 0.5 respectively. The probability that both A and B simultaneously is 0.12. then the probability that neither A and nor B occurs is
SOLUTION
Solution : D
P(A⋃B)=P(A)+P(B)−P(A⋂B)=0.25+0.5−0.12=0.63
Therefore required probability = 1 - P(A intersection B) = 0.37
Question 7
If (1+4p)4,(1−p)2,and (1−2p)2 are the probabilities of three mutually exclusive events , then the value of p is
SOLUTION
Solution : A
1+4p4,1−p2,1−2p2 are the probabilities of three mutually exclusive events.
∴0≤1+4pp≤1,0≤1−p2≤1
0≤1−2p2≤1and0≤(1+4p4)+(1−p2)+1−2p2≤1
∴−14≤p≤341−≤p≤1,12≤p≤12andp≤52
⇒p=12
Question 8
A father has three children with at least one boy. The probability that he has two boys and one girl is
SOLUTION
Solution : B
Let A be the event that father has at least one boy and B be the event that he has 2boys and one girl.
P(A)=P(1boy,2girl)+P(2boy,1girl)+P(3boy,nogirl)=3C1+3C2+3C323=78
P(A⋂B)=P(2 boy,1 girl)=38
Hence P(BA)=P(A⋂B)P(A)=37.
Question 9
A fair coin is tossed n times and let X denotes the number of heads obtained. If P(X = 4), P(X = 5), and P(X = 6) are in A.P., then n is equal to
SOLUTION
Solution : B and C
Probability of r successes in n trials is equal to nCrprqn−r, where p is the probability of success in one trial and q is the probability of failure in one trial.
This means nC4(12)n,nC5(12)n,nC6(12)n are A.P.
⇒2×nC5=nC4+nC6
⇒2×n!(n−5)!5!=n!(n−4)!4!+n!(n−6)!!
⇒6×5+(n−4)(n−5)=2×6(n−4)
⇒n2−9n+50=2(6n−24)
⇒n2−21n+98=0
⇒n=7 or n=14.
Question 10
One hundred identical coins, each with probability p of showing up heads are tossed once. If 0 < p < 1 and the probability of heads showing 50 coins is equal to that head showing 51 coins, then the value of p is
SOLUTION
Solution : D
Let P1 and P2 be the respectively be the probability of heads showing 50 coins and head showing 51 coins P1=100C50 p50(1−p)50 and P2=100C51 p51(1−p)49
Given P1=P2
⇒100C50 p50(1−p)50=100C51 p51(1−p)49
⇒1−pp=5051⇒p=51101.