Math is very functional. Most of us use it without even thinking about it. Do you ever go shopping? How do you determine how much of a product to buy? You probably take into account how many will be eating, as well as how much you'd like each person to have. If you're feeding a family of eight, perhaps more than a single box of macaroni and cheese would be in order, unless, of course, everyone is on a strict diet.
How do you know if bigger is better? Lots of products come in "value sizes", usually with a higher price tag, yet with a bigger volume. Simple math helps you determine whether or not you truly are getting more for your money.
Are "combo meal deals" really a better option at restaurants? Why pay 3.99 for a so-called value meal at Mickey D's, when you can get the sandwich, fries, and drink for only $1 a piece?
Math is used all the time. The reasoning and logic behind it help us avoid getting ripped off.