Posted by **Katia** on Monday, December 13, 2010 at 7:34pm.

my problem is:

From the box containing 5 red balls, 7 blue balls, 10 black balls. you select 4 balls

what is the probabilty that EXACTLY 2 balls are black.

so i know how to start it

c(10,2)c(2,1)

----------

c(22,40

but how do you solve that ?