Divisibility by 3, Divisibility by 9

During math tutoring, no matter what the level, the front line is arithmetic.

In that spirit, here’s a handy trick to tell if a number is divisible by 3:

Add up the digits.  If the sum is divisible by 3, then so is the original number.

Example:  consider the number 222.  2+2+2=6.  Since 6 is divisible by 3, so is 222.  (Many people might not expect that.)  In fact, 222 ÷ 3 = 74.  (Check for yourself.)

A similar law works for 9:  if the digits add up to something divisible by 9, then the original number is also divisible by 9.

Example:  consider 414.  4+1+4 = 9, so 414 is divisible by 9.  So is 864, since 8+6+4=18, and 18 is of course divisible by 9.

Of all the math tricks I know, these are two of the most useful.

Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.

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