Uploaded March 2010 | Updated September 2026, 3 weeks ago

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Most everyone seems to know the rule for divisibility by three: add up the digits and see if the sum is a multiple of three. Most seem to "know" that the same rule holds for nine.

But why do these rules work? And did you know that summing the digits tells you a bit more than just a yes/no answer?

Here is all the math explained - swiftly and easily.