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

First, determine how much water there already is in the tank:
2/3*10.5=7

Then, subtract that number from the total capacity of the tank:
10.5-7=3.5

Other method:
The number of gallons it takes to fill up the rest of the tank:
3/3-2/3=1/3

You need 1/3 of the total capacity of the tank:
1/3*10.5=3.5