Answer: 6 gallons
First we are told that we initially had 1/3 of the tank and when 10 gallons were added we had 3/4 of the tank:
(1)
(2)
(3)
This means 1 gallon is equivalent to 1/24 tank.
On the other hand if we have 3/4 of a tank and need to reach 1 tank we have to add another fraction of the tank:
(4)
(5)>>>>This is the fraction needed to complete 1 tank
Now we need to know how much that 1/4 tank is equivalent in gallons.
From (3) we know
. So, we can write the following relation to transform from fractions of a tank to gallons:
Therefore:
6 more gallons are needed to fill the tank