Divisibility Rules for 3 and 9 (VIDEO!)
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.