How do you know whether a number is divisible by 3?

Let’s take an example 154 642 587 984

Here’s the long way –

Add the numbers up. Total = 63

Add these numbers up Total = 9

If 9 is divisible by 3 (which, of course, it is), then the number that we started with is divisible by 3.

Here’s the quick way –

Starting from either end discard any sets of numbers that are divisible by 3 or which add up to a number that is divisible by 3.

15 4 6 42 5 87 9 84

Add the remaining numbers 4 + 5 = 9 and that’s divisible by 3, so the original number is divisible by 3.

Why does it work?

I have no idea!

What use is it?

I have no idea!