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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