Please help me figure out this problem! I just can't figure it out!

A 4-person grievance committee is to be composed of employees in 2 departments, A and B, with 15 and 20 employees respectively. If the 4 people are selected at random from the 35 employees, what is the probability of selecting:

1) 3 from A and 1 from B

2) 2 from A and 2 from B

3) All from A

4) At least 3 from A