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April 6, 2009


There are 19 red balls and one black ball. Ten balls are put
in one jar and the remaining 10 are put in another jar.
What is the possibility that the black is in the right jar.


Secret Squïrrel said...

Since it has an equal probability of being in either jar, the chance of it being in the right one is 50%. This will always be the case as long as there are the same number of balls in each jar (and only one black one).

pannag said...

It can be viewed like this:
The probability that the Black ball is in the first jar
= 19C9 / 20C10 = 1/2.

(there are 20C10 ways of choosing any 10 balls. Out of that there are 19C9 ways in which the Black ball is one of them).