Playing time (1/2): When is a number divisible by 3? (Are you up for the challenge?)

Manolo

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At school, we learned that if the sum of its digits (cross sum) is divisible by 3, then the number itself is divisible by 3. A new method for Pros can be executed entirely mentally, leaving everyone astonished when you use this trick. It works regardless of the number's size. Here's a small example number, and it's possible to apply it to infinitely larger numbers: 73646229385734472654132980978732023023446763338576056857.