P5 learn about ‘digital roots’ (repeated digital sum) to work out whether a number is a multiple of a certain times table.

How do you know a number is a multiple of six?

Step 1 –

The digital root is 3, 6 or 9.

e.g. Is 366 a multiple of 6? 3 + 6 + 6 = 15

Step 2 –

Take that number 15.

1 + 5 = 6, so 366 is a multiple of six.

Likewise –

How do you know a number is a multiple of nine?

The digital root is 9, e.g. 369 = 3 + 6 + 9 = 18  1 + 8 = 9,
so 369 is a multiple of nine.

How do you know a number is a multiple of three?

The digital root is 3, 6 or 9. e.g 369 = 3 + 6 + 9 = 18  1 + 8 = 9, so 369 is a multiple of 3.)