How do you know if a number is a multiple of 7?

Answer 1

There is a trick to figuring out whether a number is divisible by 7, without having to divide it directly. Unfortunately, the 'shortcut' is almost as hard as simply doing the division in the first place.

But for those who want to know, here's the trick:

1. Take the final digit of the number.
2. Double it.
3. Subtract it from the rest of the digits.
4. Repeat until you get a number that is (or isn't) a multiple of 7.

For example, try 98:

1. The final digit is 8.
2. 8 x 2 = 16.
3. 9 (98, without the 8) - 16 = -7.
4. -7 is a multiple of 7, so 98 is divisible by 7.