Here is the correct answer, and explanation for how to get the answer, and a way to remember how to use them for next time :)
There are 4 quarts in a gallon so the amount used can be shown as 4 1/4, or 4.25 so to find out how much gas is left in the tank you simply subtract 4.25 from 20.
20-4.25
=15.75
There is 15.75, or 15 3/4, gallons of gasoline left in the tank.
3/8 = 1/8 +1/8 + 1/8 ; 3/8 = 1/8 + 2/8 ; 2 1/8 = 1 +1 + 1/8 = 8/8 + 8/8 + 1/8.
Hope this helps!
The answer would be (B/A)*100%