Hello there!
Let's look at all the information given to us.
- The tank can hold 13.5 gallons
- It is 2/3 full
And now the question it's asking.
- How many gallons to fill it?
Essentially, it's asking how to make the tank 100% full when it's already 2/3 of the way full.
To solve:
What fraction of the tank needs to be filled up?
100% is also equal to 3/3.
The difference between 2/3 and 100% therefore, can be rewritten as the difference between 3/3 and 2/3. Then it's clearer that the amount the tank needs to be filled is 1/3 of it's maximum capacity.
The maximum capacity is 13.5 gallons.
What's 1/3 of 13.5? To get that, multiply 1/3 by 13.5, to get 4.5.
Therefore, 4.5 is 1/3 of 13.5, and is therefore the amount of gasoline the tank needs to be filled up.
Your answer is 4.5 gallons.