11/13/01

One of my favorite tricks is that I can instantly tell if a number is divisible by six.

It’s easy. Here’s how in two steps:

1. It must be even. If it’s not even, it’s not divisible by six, which is even.

2. It must be divisible by three. Here’s how to tell if a number is divisible by three: add up all the digits. If the sum of the digits is divible by three, then the number is divisible by three.

Examples:

5487: no, not even.

368: even, but not divisible by three, since 17 (the sum of the digits) is not divisible by three. So it’s not divisible by six.

8796: even and divisible by three—hence divisible by six.