1+5+4 = 10 (even digit sum) For small values of X < 10 11X = X+X, 2X always even For slightly larger values of X, where 11X < 200 11x10 = 110, (starts out e

1+5+4 = 10 (even digit sum) For small values of X < 10 11X = X+X, 2X always even For slightly larger values of X, where 11X < 200 11x10 = 110, (starts out even) 11x11 = 121, (both of the first two digits increment by one, flipping even or odd ... in unison) 11x18 = 198, (only two signs will flip until we increment the 100s place, when that happens we will reverse three signs instead of two) 11x19 = 209, (now, three instead of two flip, and our sum comes up odd for the first time, 2+0+9=11)