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GMAT Problem Solving (PS) | Re: Twenty people at a meeting were born during the month of Sep

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vad3tha wrote:

kennaval wrote:
Twenty people at a meeting were born during the month of September, which has 30 days. The probability that at least two of the people in the room share the same birthday is closest to which of the following?
(A) 10%
(B) 33%
(C) 67%
(D) 90%
(E) 99%



The probability that at least two people sharing the same birthday = 1 - the probability that none of them sharing the same birthday
A =The number of ways of none of them sharing the same birthday = 30P20 = 30!/(30-20)! = 30!/10!
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