Dear Mary: I am university-educated and have an advanced degree in the arts. This is really embarrassing: What is interest? I just don’t get it. —Marty Dear Marty: Don’t worry. Your secret is safe with me. There are lots of things in the arts that I don’t understand, so that makes us even. OK, let’s say you’re a photographer. You have a big job offer, which will require you to borrow a camera. You’ll have to pay a rental fee, figured by the hour or the day. The longer you keep the camera, the more rent you will pay. Interest is like rent on money you borrow. Now let’s say that, instead of you borrowing a camera for the day, you borrow $1,000 from your credit union. Each month that you do not return that borrowed $1,000, you must pay a kind of rent for using it, otherwise known as …