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.