Assistance with Permutations

Question: There are 18 mathematics majors and 325 science majors at a college.
a. how many ways can 2 reps be picked so that 1 is a mathematics major and 1 is a science major?
b. how many ways can 1 rep be picked who is either a mathematics major or a computer science major?

I am having a hard time finding the answer. If I could get some assistance, that would be great.

a) Choose a math major and choose a science major (order is assumed to be irrelevant). How many ways can you do that? (hint: there is a basic rule about counting # ways that applies here)
b) No info about how many computer science majors there are