What are all the methods of x 19 and 21?

Answer 1

It is probably impossible to enumerate all the methods.

Apart from long multiplication you can have

Two-way partitioning:

(10 + 9) * (20 + 1) = 200 + 10 + 180 + 9 = 399

This is usually done in a 2x2 grid.

Partitioning 21

19*(20 + 1) = 19*2*10 + 19*1 = 38*10 + 19 = 380 + 19 = 399

Partitioning 19

(20 - 1)*21 = 2*10*21 - 1*21 = 42*10 - 21 = 420 - 21 = 399

Or. if you are savvy enough,

Difference of two squares

(20 - 1)*(20 + 1) = 202 - 11 = 400 - 1 = 399