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probability that exactly 2 will buy

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by vittalgmat » Thu Jan 01, 2009 1:56 am
The probability that a visitor at the mall buys a pack of candy is 30%. If three visitors come to the mall today, what is the probability that exactly two will buy a pack of candy?


A. 0.343
B. 0.147
C. 0.189
D. 0.063
E. 0.027


[spoiler]OA: C[/spoiler]

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by piyush_nitt » Thu Jan 01, 2009 3:32 am
Probability that visitor buys candy P(A)= .3
Probability that visitor NOT buys candy P(!A) = .7

Probability that Exactly 2 visitors buy candy (say A, B, C are visitors) =

P(!A)P(B)P(C)+ P(A)P(!B)P(C)+ P(A)P(B)P(!C) = .7.3.3+ .3.7.3+.3.3.7 = .189
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Re: probability that exactly 2 will buy

by logitech » Thu Jan 01, 2009 11:01 am
"two will buy a pack of candy" BUY BUY NOTBUY can be selected in 3C2=3 ways

each time it is 3/10 x 3/10 x 7/10 ( 1-[3/10] = 7/10 the probability of NOT buying )

So we will have 3 scenaroius with the same probability

3 x 3 x 3 x 7 / 10 x 10 x 10 = 9/10 x 10 x 21 /10 = 9x2.1/100 = 0.189
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