Convert both denominators to be the same number. To do that, we need to figure out a number that has both 5 and 20 as factors. 20 itself has both of those numbers as factors.

(4 * 4)/(5 * 4) = 16/20

Since 4/5 is 16/20, when you compare it to 19/20, you will see that 19/20 is greater.