And you can apply that same sort of logic in your head to just about any number, though of course with larger numbers it gets trickier.

Say, 13 x 18, you take ten 18s (180) and add them to three 18s (54) to get 234. You can do that for other two-digit numbers as well. Say, 72 x 44, first you take seven 44s, which to make easier in my head I break down to two 132s (3x44) plus 44, so seven 44s is 308, add a zero, 3080, then add two 44s, or 88, for a final mental math score of 3168. That's no longer as helpful when you get to 1377 x 8513, but that's what paper is for.