Mental Calculation of Multiplication Base 10

n x r = (n + (r - 10)) x 10 + (n - 10) x (r - 10)

From

n x r = (n - 10) x (r - 10) + (10n + 10r - 10^2)

example

13 x 12 = ((13+(12-10)=15)x10 = 150) + ((13-10)x(12-10) = 6) = 156

14 x 17 = ((14+(17-10)=21)x10 = 210) + ((14-10)x(17-10) = 28) = 238

17 x 9 = ((17+(9-10)=16)x10 = 160) + ((17-10)x(9-10) = -7) = 153

8 x 18 = ((8+(18-16)=16)x10 = 160) + ((8-10)x(18-10) = -16) = 144