“Pick a number from 1 to 100.”

Today I’m thinking about the number: 57.

What can be said about the number fifty-seven?

It almost looks like it might be a prime number (due, I expect, to the five and the seven, both of which are prime). But it’s not prime (or in its prime). It’s divisible by 3, and that’s easy to see, because:

That trick works for any number divisible by three. Just keep reducing the digits by adding them until you get a single digit. If that digit is divisible by 3 (3, 6 or 9), then the number can be divided by three. We could even have stopped at the 12, since that is also divisible by 3.

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