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Originally Posted by blazi
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You have to think of two digits, let's call them x and y. So the number will be 10x + y. If x = 2 and y = 3 that is 23. The sum is x + y, in this example 5. Subtract from the two digit number and you get 18. Now assume you thought of y = 4. That would be 24 and 6, where 24 - 6 is also 18.
Let's do the algebra. (10x + y) - (x + y) = (10x - x) + (y - y) = 9x.
It does not matter what y is. You always get a number that can be divided by 9. Now look at the website. All of these have the same symbol. 0, 9, 18, 27 ...