The why of the 9 divisibility rule (Full video)

Khan Academy

Supporting grades: 6

Description: Why you can test divisibility by 9 by just adding the digits. Created by Sal Khan. To figure out whether it's divisible by nine, I just have to add up the digits and figure out if the sum of the digits is a multiple of nine or whether it's divisible by nine. But this might leave you thinking, well, that was nice and useful, but why did that work? So the two in 2943, it's in the thousands place, so we can literally rewrite it as two times, two times 1000, the nine is in the hundreds place, so we can literally write it as nine times 100, the four is in the tens place, so it's literally the same thing as four times 10, and then finally we have our three in the ones place, we could write it as three times one or just three.

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