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 redneckmath.blogspot.com and okieinexile.blogspot.com. You may contact him at firstname.lastname@example.org. We invite you to “like” the National Association of Lawn Mowers on Facebook.)