Math is important and math teachers have a hard job. It’s not just teaching math; it’s what math you should teach. Should you teach the students who are going to design computers, the ones who are going to wash your car, or the ones in between? As a teacher, you don’t have a choice; you’ve got to teach all of them. And it would be nice if they learned something useful.

I am a mathematician, but I don’t really know any useful math, just the theoretical stuff. However — out of shame — over the years I’ve learned one or two tricks about doing practical arithmetical calculations.

For example, say you take the clan out for a bite at Chili’s. The bill comes and it’s $84.12. It’s a chunk, but you don’t eat out that often. You are about to leave a 10 percent tip because it’s easy to figure, but your daughter informs you that 10 percent is not enough. Some of her friends wait tables and they need the tips, so you have to leave at least 15 percent or let your daughter die of terminal embarrassment, but you don’t have a calculator. What do you do, what do you do?

One answer is to plop down $10 and forget about it, relying on the fact you daughter doesn’t know how to do percentages either. There is another way, however. Ten percent of 84.12 is 8.40. Yes, I know it’s 8.41, but if a penny is going to save their life, they are going to die anyway. Five percent is half of 10 percent and that is 4.20. Add those together and you get 12.60, i.e. 15 percent. Round it up to $15 because the waitress is cute. And when you reach the age of 50, they are all cute by the way.

Say you’ve had a glass of wine or two and those decimals are blurry. Then there are two ways to do this. If the waitress is cute — and it can’t be repeated often enough that they all are — then say $84.12 is about $90. Ten percent of that is 9 and half of that is 4.50. That’s about 5. Five and 9 is about 14. Round it up to $15 because she’s cute. Now suppose the waitress is not cute. That is to say, it’s a waiter, a young man who called you gramps. Then say $84.12 is about $80. Ten percent of that is 8 and half of that is 4. That comes to 12. You’ve got $10 but no ones so you’re about to put down a ten, but your daughter gives you the look, so you plop down a five alongside it.

So it turns out the math of the thing is the simple part. All of the sociological/familial considerations and judging the cuteness of the waitress are hard.

Oh, we suffer.

The reason that the math is simple comes from two things. Because of our base 10 numeration system, taking 10  percent is easy. Halving numbers is fairly easy as well. This process is made even easier by rounding. Rounding is not laziness; it is simply efficiency. You don’t want to use up too much of your dining-mate’s time with your mathematical prognostications. And when you are dealing with coins and currency, being exact is problematic anyway as you have to work with the denominations you have on hand.

Judging the cuteness of waitresses is another matter. If I delved into it too deeply in this space, I would soon be paying a lawyer or two to figure out what half of my assets is, and that is painful regardless of the math.

(Bobby Winters, a native of Harden City, Oklahoma, is Associate Dean of the College of Arts and Sciences and Professor of Mathematics at Pittsburg State University. He blogs at and You may contact him at We invite you to “like” the National Association of Lawn Mowers on Facebook.)


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