SOLUTION: Box A contains 5 red balls and 4 white balls. Box B contains 6 red balls and 5 white balls. If one ball is selected at random without looking at the boxes and is found red, what is

Question 1194868: Box A contains 5 red balls and 4 white balls. Box B contains 6 red balls and 5 white balls. If one ball is selected at random without looking at the boxes and is found red, what is the probability it came from box A?
Found 2 solutions by Boreal, greenestamps:
Answer by Boreal(15235) (Show Source): You can put this solution on YOUR website!
There are 20 balls altogether, and 11 are red.
If a red ball is found it has a 5/11 chance coming from box A
======R========W======Total
A------5-----------4----------9
B------6-----------5---------11
Total--11-----------9---------20
If we select a red ball, it has a 5/11 chance of being from box A.
Answer by greenestamps(13200) (Show Source): You can put this solution on YOUR website!

Note that the number of white balls, and the total number of balls, is irrelevant for the question that is asked; we only need to know the number of red balls in each box.

There are 5 red balls in box A and 6 in box B, so when a red ball is selected, the probability that it came from box A is 5/(5+6) = 5/11.

ANSWER: 5/11

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