The difference between 2 numbers wil the same digits will always be a multiple of 9. Consider a 2 digit number as 10x + y. Transpose the digits; 10y + x. (10x + y) - (10y + x) = 9x - 9y.

## Multiple of 9

The difference between 2 numbers wil the same digits will always be a multiple of 9.

Consider a 2 digit number as 10x + y. Transpose the digits; 10y + x. (10x + y) - (10y + x) = 9x - 9y.