Okay so you need to start by making an equation for yourself. Since the container starts at 1/10 of the total, you want to start by saying 1/10x. Then you add 12 gallons of water to make it half full. So 1/10x + 12 = 1/2x where x is the total amount that can fit in the container.
1/10x + 12 = 1/2x (Move your x's to the same side.)
12 = 1/2x - 1/10x (Find a common denominator.)
12 = 5/10x - 1/10x (Simplify/)
12 = 4/10x (Multiply both sides by 10; your demoninator)
