[zellux]

The trick applies to two-digit multiplication xy * xz while y + z = 10. And we can simply calculate like this:

  34 * 36 = 3*4 * 100 + 4*6 = 1224
  57 * 53 = 5*6 * 100 + 7*3 = 3021

For multiplications like yx * zx as long as y + z = 10, we need to add x * 100, like this:

  24 * 84 = 2*8 * 100 + 400 + 4*4 = 2016
  46 * 66 = 4*6 * 100 + 600 + 6*6 = 3036