What is 15 squared?

Fifteen squared is 225, and there is a shortcut that makes every number ending in five easy to square in your head. Take the digits before the five, multiply that number by the next whole number up, and stick 25 on the end. For 15, take the 1, multiply by 2 to get 2, then append 25 for 225. For 25, take 2 times 3 to get 6, giving 625. For 35 you get 3 times 4 equals 12, so 1225, and the pattern never fails. It works because a number ending in five can be written as ten times something plus five, and expanding that expression algebraically produces exactly this rule. Squares in general are worth practising because they let you multiply nearby numbers quickly: 14 times 16 is 15 squared minus one, or 224, using the difference of two squares. The trick turns a slow multiplication into two small steps.

Every square number ends in 0, 1, 4, 5, 6 or 9, so anything ending in 2, 3, 7 or 8 cannot be one.