I'm a mathematician and I want to show you how to multiply 20x20 in your head. You can use it to impress your friends.
So now the trick:
You take 15 x 13 as example. you place the larger number on top.
First add 15 + 3 (last number of 13) = 18. then multiply it by 10= 180.
Multiply the 3 and "5" (3x5= 15). Then you summ it 180 + 15 = 195.
Practice! It's easy!