HI Nat,

I think you mean 1200 cubic centimeters $(cm^3),$ not square centimeters.

The volume of the water in the tank is the area of the base times the height, ie $A \times h$ where $A$ is the area of the base in square centimeters and $h$ is the height in centimeters. You know the area of the base is 80 square centimeters so solve $A \times h = 1200$ for $h.$

This is the height of the water which is $/large \frac{5}{9}$ of the height of the tank. What is $\large \frac{1}{9}$ of the height of the tank?

Penny