(X)(Y) = (N-X)(N-Y) + N(X+Y-N)
This is representation of how we used to teach children multiplication on their ten fingers by overlapping two numbers from opposite sides and multiplying the resulting left with right, and adding that to the number in the middle (multiplied by ten).
My question, is there values where N does not apply?
http://lifehacker.com/5942037/quickly-multiply-single-digit-numbers-with-your-hands