Question

A 13.5-gallon gasoline tank is 4/5 full. How many gallons will it take to fill the tank?

Answer 1

2.7 gallons. 
 Since the tank is already 4/5 full, only 1/5 is needed to fill it completely. As 13.5 gallons represent 5/5 (the full tank), you can take 13.5 divided by 5 to find 1/5.