Some years later, I got married. My wife, Leila, is especially good at math. We would sometimes go out to dinner in restaurants. When it came time to pay the bill, I always struggled to figure out the correct amount to leave as a tip for the server---usually 15 percent of the total. Leila used to look at me in amusement. Finally one night, she said, “Tell me what process you go through when you are trying to calculate the tip.” 6I said, “OK. I want to leave a 15 percent tip. So first, I take the total amount of the bill, and then I divide it by 10 in order to figure out 10 percent.