Approximate what a 15% tip would be for a bill of $31.99.

Use this trick: to get 10% of anything, divide by 10 (move the decimal place one spot to the left).

Since it's an approximation, use 30 instead of 32 to make it easier.

30 / 10 = 3.0

So 3.0, or just 3, is approximately 10%. Divide by 3 by 2 to get 5%.

3 / 2 = 1.5

Since we know 10% is 3 and we know 5% is 1.5, we can add them together to see that 15% is 3 + 1.5 = 4.5 (approximately)