Question 626854
The product of the sum and difference is equal to the difference between the squares. 
In symbol: {{{(a+b)(a-b) = a^2 - b^2 }}}. 
Let a = 20 and b = 1 so {{{(20+1)(20-1) = 20^2 - 1^2 = 400 - 1 = highlight(399) }}}