If it’s less than $5 it’s basically free. If you return an item for a refund, you’ve made money. If you paid for a vacation months ago, it’s free by the time you go on it. And if you buy something ...

I offer to pay you $200 in one year if you give me $190 today. Good deal or bad deal? It’s the kind of math problem you might encounter in real life, as opposed to, say, whether the cosecant of a ...