Question

A 10.5-gallon aquarium is 2/3 full. How many more gallons of water does it take to fill the aquarium?

Answer 1

The answer is 3.5 gallons

Answer 2

First, you need to find out how much water is already in the tank. 
10.5 * 2/3. 
And then you subtract the total amount and how much is already in 
10.5-7 = 3.5