A 13.9 gallon gas tank is 4/5 full. How many gallons will it take to fill the tank?

It will take 2.78 gallons to fill the remainder of the tank.

13.9 * 4/5 = 11.12 - 11.12 is the amount of gas already in the tank.

13.9 - 11.12 = 2.78 - So, subtract 11.12 from 13.9 and you get your answer.

Hope this is right!; )