Multiplying by 5, 50, 500

Mental Math · Axiom Academy

LESSON Multiplying by 5, 50, 500 Five is exactly half of ten, and multiplying by ten is free — so multiply by ten and take half. 1. Ten Is Free, and Five Is Half of Ten Ten is the one multiplier nobody has to think about. 48 × 10 = 480 — every digit simply moves up a place, and there is nothing to remember. Five is worth exactly half as much as ten, so five of something must be half of ten of them. So take the free step first. Ten of the 48s is 480; half of 480 is 240 . That is your answer: 48 × 5 = 240 . Notice what you never did — you never recalled a five-times fact, and you never carried anything. You performed one free operation and one halving. Multiply by ten, then take half For 36 × 5 : ten of them is 360, and half of 360 is 180 . The two moves can also be taken in the other order — half of 36 is 18, and 18 × 10 is the same 180. Both orders are correct, because you are doing the same two things either way. 2. Odd Numbers: Stretch First, Then Halve Halving is effortless right up until the number is odd. Halve 37 and you get 18.5 . Nothing has broken — 18.5 × 10 = 185, which is the correct answer — but you are now carrying a decimal in your head, and that is exactly the sort of thing that goes wrong at speed.

This is the written version of the interactive lesson above. See the full Mental Math course.