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The probability that a visitor at the mall buys a pack of

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The probability that a visitor at the mall buys a pack of [#permalink] New post 14 Jun 2008, 08:18
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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?

0.343
0.147
0.189
0.063
0.027

Please explain.
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Re: Probability - Candy Buying [#permalink] New post 14 Jun 2008, 08:23
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IMO the answer is c i.e .189 ...

2 people out of 3 can buy candies in 3 ways ( person 1+2 , person 1+3 , person 2+3)

now probability of 2 person buying a candy and 1 not buying is .3 x .3 x .7

this can happen in 3 ways

so the combined prob is 3(.3 x .3 x .7) = .189
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Re: Probability - Candy Buying [#permalink] New post 14 Jun 2008, 09:08
Why do we need to consider the number of ways (3) it can happen?
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Re: Probability - Candy Buying [#permalink] New post 14 Jun 2008, 10:10
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cos the stem says 30% for one person

Take 3 persons A B and C

Scene 1 A and B buy candy but C does not

then prob is (.3 x .3 x .7)

Scene 2 B and C buy candy but A does not

prob again is .3 x .3 x . 7

Scene 3 same probablity for A and C buying but not B

Scene 1 , 2 and 3 is an 'or' situation in probablity as all the 3 scenes satisfy the main stem condition that 2 ppl buys and one does not

so u will have to add the probability of scene 1 + prob of scene 2 + prob of scene 3 ( basic rule of 'or' situation)

so what u essentialy get is 3 ( .3 x .3x .7) or (.3 x .3 x .7 ) + ( .3 x .3 x .7) + (.3 x .3 x .7)

hope that explains
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Re: Probability - Candy Buying [#permalink] New post 15 Jun 2008, 05:27
maulik - Thanks for the detailed explanation. +1 to you.
Re: Probability - Candy Buying   [#permalink] 15 Jun 2008, 05:27
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