Stuck on a probability question please help!!!!!
A state lottery begins with choosing a ball from 30 table tennis balls that are numbered 1-30 in a bin. The first ball is put on display, then a second ball is chosen.
a) what is the probability that a 4 is chosen, then a 31 is chosen?
b) What is the probability that a 2 or 13 is chosen, then a 1 is chosen?
I know the right answers are a) 0 and b) 2/435 but I don't know how to get those answers.