The trick is even more nifty: You do not need to sum up all the digits. You can ignore all 3s, 6s, 9s and 0s. And: If it is divisible after the second summary it is not only divisible by 3 but by 9 too!Flumble wrote:Indeed. And the nifty trick to find out if a number is divisible by 3 is to sum the digits and check if that is divisible by 3. (and if that's still hard, sum those digits and check if that is divisible by 3 etc.)

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