I have one math question left and I can't figure out how to set it up and solve it?

The question is this: Jonah has a large collection of marbles. He notices that if he borrows 5 marbles from a friend, he can arrange the marbles in rows of 13 each. What is the remainder when he divides his original number of marbles by 13? Serious answers only. It's the last question and I've been sitting on it for 2 days. Thanks!