Please tell how you arrived there soi understand it, that is the most imporant thing.
How many ways can sierra select 3 movies to rent from the top 10 rentals?
A.720 ways, B.240 C. 90 ways D.120 ways
I think it's 10 chose 3. That is, . To do it, I didn't think a lot. I mean the formula I gave you is very well known and useful in cases like yours here. It means, if you have 10 elements, how many ways are they if you want to pick 3 of them? This is exactly your problem. So I guess you'll get used to it after having done every kind of problems.
More generally, if there is a total of n element and you want to pick up m elements, the total possibilities are . You should learn this formula and check out your notes, it is probably there and well explained.