They also add up to a multiple of 9.
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That they add up to 9
If they are multiples of 9, the digits add up to a multiple of 9.
Because they're in multiples of three - a quick way to tell if a number is a multiple of three is to add up the digits and see if the digits add up to a multiple of three e.g 576, 5+7+6=18, 1+8=9, 9 is a multiple of three
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