[zeuzurcz] Multiplication table up to 20

One only needs to memorize the multiplication table up to 5*5 in order to be able to multiply up to 10*10.  For digits larger than 5, substitute (10-x) and use the distributive rule (FOIL).

However, having sunk the cost of memorizing up to 10*10, one can apply the same principle to multiply up to 20*20.  Recursively go further, i.e., 40, though that might be more work than the traditional tableau.

Flash cards app showing partial sums.