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! ;)
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