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If the sum of digits of a number add to a multiple of 9, then the original number must be a multiple of 9.

You can multiply by nine on you fingers if the number is less than 10. Just hold up all your fingers (and thumbs) and put down the finger the represents the number you are multiplying. Lets say we want to multiply 7 by 9. Just put down finger 7. You will have 6 fingers to the left of finger 7 and 3 on the right, giving you 63. Or use 9. Put down finger 9, and you have 8 to the left and 1 to the right. Yes, 9 times 9 is 81.

The rule given at the top is why the finger trick works. You have 10 fingers, and if you put one down, you have nine. When multiplying any number between 1-10 by nine, adding the the digits of the product always gives nine - the same number as there are fingers left.

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