# Multiplication by 9, 99, 999, etc.

One way to multiply a number by 9 is to multiply by 10 and then subtract the number from the product. For example,

23·9 = 230 - 23 = 207. |

The rule is based on the distributive law and applies in other similar situations. For example, to multiply by 99, multiply by 100 and then subtract the number:

23·99 = 2300 - 23 = 2277.

To multiply by 999, multiply by 1000 and subtract the number from the product:

23·999 = 23000 - 23 = 22977.

|Contact| |Front page| |Contents| |Algebra| |Math magic|

Copyright © 1996-2018 Alexander Bogomolny

66157183