

How to Multiply numbers by 9, 99, 999, 9999 and so on in less than 5 seconds - mquaes
http://mathema-tricks.blogspot.com/2012/03/multiplying-numbers-by-9-99-999-9999.html#links

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twiceaday
Why not simply show the formula?

x * (10^n - 1) = (x - 1) * 10^n + (10^n - x)

