my problem is:

Question

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 ?

Answer 1

i meant c(10,2)c(2,1) divided by c(22,4)

Answer 2

so we want 2 from the black, then 2 more from the rest
prob of that = C(10,2) C(12,2) / C(22,4)