**Divisibility by 9**:

- Add up all the digits in the number,

- If answer is divisible by 9, then the original number is divisible by 9.

__Concept__:

Let's take a 4-digit number, abcd. Re-writting it, 1000a + 100b + 10c + d.

It can also be re-written as 999a + 99b + 9c + a + b + c + d.

Look at the left-most 3 terms (999a, 99b, 9c), they are surely divisible by 9.

Thus, it leaves us with the a + b + c + d to determine whether the original number can be divided by 9. This a + b + c + d is the rule for divisibility by 9.

To view other concepts of number divisibility,

**click here**.

:-)

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