Probability

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jessicamuniz: Junior | Next Rank: 30 Posts; Posts: 26; Joined: Thu Jul 17, 2008 1:02 pm

by jessicamuniz » Tue Dec 02, 2008 9:17 am

Hello everyone,
Here is a question:
Box1 has 3 red balls and 5 white balls. Box2 has 4 red balls and 2 whites. One ball is randomly choosen from the Box1 and put it in Box2 without seeing the color. After that, another ball is choosen from the second box. What is the porobability that this ball were white?

ANS: 21/56

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Source: Beat The GMAT — Problem Solving | Tue Dec 02, 2008 9:17 am

GMATters1001: Senior | Next Rank: 100 Posts; Posts: 33; Joined: Mon Nov 03, 2008 1:55 pm; Thanked: 3 times

by GMATters1001 » Tue Dec 02, 2008 9:40 am

There are 7 balls in box two when the ball is chosen. 2 of them are definitely white. One of them has a 5/8 chance of being white. So the probability that the ball picked out of that box is white is:

2/7 + (1/7)*(5/8) = 16/56 + 5/56 = 21/56
simplified would be 3/8 by the way.

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