I missed part of my last stats class and don't understand the homework at all so I was hoping someone here could help me understand it...

The question is on blood groups and Rh types of 100 subjects and table shows this..

O A B AB

Rh+ 39 35 8 4

Rh- 6 5 2 1

And the question is:

If 3 of the 100 subjects are randomly selected, find the probability that they are all group B and type Rh-.

a) Assume that the selections are made with replacement.

b) Assume that the selections are made without replacement.

So I know that with replacement the sample size doesn't change but without replacement it decreases, but that is all i know for setting up this problem.

I appreciate any help I can get! Thanks!