I 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. Then, I take the amount that is 10 percent, and I divide it by two and that gives me 5 percent. Then, all I have to do is add the amount that is 10 percent to the amount that is 5 percent, and I get 15 percent.” I thought I had explained things rather well.