Answer:
30 Gallons
Explanation:
Ok, lets see the difference the 9 gallons of gas did to Rafael's gas tank.
1/2 --> 0.5
4/5 --> 0.8
0.8-0.5 = +0.3
The tank was made 30% more full with the gas.
Next, we can use this to find the amount of gas which would need to be used to fully fill the tank, given that it was empty.
So to make it easier, say we filled it up a third of 30% to get an easier to work with 10% of fill of the gas tank.
9 / 3 ---> 3 Gallons
Now, multiply this by 10 to get the 100%
3 * 10 = 30 Gallons
All done!