probability
2. Suppose your friend places—with equal probability—either 10 red
balls and 10 green balls or 5 red balls, 10 green balls, and 5 yellow balls in a box (in particular, the box contains 20 balls). You draw five balls from the box uniformly-at-random, without replacement.
(a) Given that 10 red and 10 green balls are in the box, what is the probability that you draw exactly four red balls?
(b) What is the probability that you draw exactly four red balls?
(c) Given that you draw all green balls, what is the probability that the box originally had 10 red balls and 10 green balls?