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Interview Question for Trader Assistant Intern at DRW Trading:
Mar 02, 2010

What is the chance that at least two people were born on the same day of the week if there are 3 people in the room?


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0 of 3 people found this helpful

Nov 22, 2010

by Anonymous:

~89.8%
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Jan 18, 2011

by Chase Schwalbach:

incorrect.

1-364/365*363/365 = 1-.9918 = .0082...something like this for 2 people, for 2+ add in the chance of 3 of 3.

So first guy then gets 365/365 choices, next two get 1/365
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Jan 22, 2011

by traderguy:

Also incorrect...

They are asking if they were born the same day of the week not the exact same day as Chase alluded to.

In this case the easiest way to do the problem is 1- probability that no one was born on the same day.

1-(1)(6/7)(5/7)+ 19/49 or 38.78%
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Jan 22, 2011

by trader guy:

My bad =19/49
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Mar 26, 2011

by K Z:

Or you can calculate it directly:
The chance of SELECTED two people(but not three) that are born on the same day is 1* 1/7 - 1/7*1/7 =6/49 Now we have 3 chooses 2 = 3 possible pairs, so the probability that any PAIR(but not all three) that are born on the same day is 3* 6/49 = 18/49. However, since the question asks "at least 2", we will need to add the possibility that all three are born at the same day, which is 1/7 * 1/7 = 1/49.
So the final result is 18/49 + 1/49 = 19/49
This obviously is a worse way of calculating it, but it's an alternative for some people who wants to think "straight".
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