Tax and Tips

A discount takes a percent away. A tax or a tip adds one. A meal costs 50 dollars and you leave a 20% tip. The tip is 20% of 50, which is 10 dollars, so you pay 50 + 10, which is 60 dollars.

On the board, one line holds it all: $t = 50+20\%\cdot 50$. The multiplication goes first, so the tip is worked out before it is added, and the next line is $t = 60$. Sales tax works the same way: 8% tax on a 25 dollar book is 2 dollars, so the total is 27.

A quick check: the total must be more than the price, but not by much when the percent is small. A 20% tip adds a fifth of the bill, so a total about 20% bigger is right, and a total that doubled is not.