Stats
posted by McKenzie .
A bag contains 10 red and 8 green marbles. 5 marbles are selected without replacement. What is the probability that exactly 2 are green?

One possible outcome is
GGRRR
the prob of that is (8/18)(7/17)(10/16)(9/15)(8/14)
= 2/51
But the 2 G's and the 3 R's can be arranged in
5!/(2!3!) or 10 ways
so prob of your event = 10(2/51) = 20/51