Question: how to add and subtract in your head?
Answer: To add 9 to another number, add 10 and then subtract 1: 36 + 9 = 36 + 10 - 1 = 45.
To add 18 to another number, add 20 and then subtract 2: 48 + 18 = 48 + 20 - 2 = 66.
To add 97 to another number, add 100 and then subtract 3: 439 + 97 = 439 + 100 - 3 = 536.