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20 April, 22:15

Which of these numbers are exactly divisible by 9? 107,513,944,126,80

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  1. 20 April, 22:42
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    513 and 126 both are. (The question doesn't say

    that there's only one of them in the list.)

    Here's an easy way to tell: (I don't know how or why it works,

    but it works.)

    You're looking at a big number, and you need to find out if it's

    divisible by 9.

    Here's the two-step process:

    - - If it's going to be divisible by 9, then it has to be divisible by 3.

    - - Find out whether it's divisible by 3. If it is, then ...

    - - Find out whether it's divisible by 9.

    Step 1:

    Add up all the digits in the number. If that sum is divisible by 3,

    then the original big number is too.

    Example:

    Is 549 divisible by 3?

    Add up its digits: (5 + 4 + 9) = 18.

    Is 18 divisible by 3?

    Yes. That tells you that 549 is divisible by 3.

    So it could be divisible by 9.

    Step 2:

    Take that same 18 and add up its digits.

    (1 + 8) = 9.

    9 is divisible by 3.

    This tells you that the original big number ... 549 ... is also divisible by 9.
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