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Suppose the two digits of the first number are x and y where 0<=x,y<10.

[Strictly speaking, if it is a two digit number then x>0, but the proof still works.]

Then the first number is 10x + y

and the second number, therefore, is 10y + x

Taking the second number away from the first gives: (10x + y) - (10y + x)

= 10x + y - 10y - x = 9x - 9y = 9(x-y)

Now, since x and y are integers, x-y must be an integer.

That is, the result of the subtraction is 9*an integer ie it is a multiple of 9.

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