The relationship between how much water is in the tank (the height of the water) and the outflow_rate is described more precisely in Torricelli's law. This law says that the velocity of water flowing out of a drain is equal to the velocity an object will have when falling from the water surface to the drain pipe. This velocity is equal to the square root of two times the gravity acceleration times the height of the water:
Where v is velocity, g is the gravity acceleration (about 9.81 m/s2), and h is height of the water in a cylindrical tank. Gravity acceleration refers to the rising speed of falling objects, given the gravity of the earth.
Outflow velocity, is not to be confused with outflow rate. Outflow velocity is how fast the water flows out, not how much! To know how much it is also important to know how big the drain pipe is. The amount of water that flows out is equal to the area (A) of the drain pipe times the velocity of the water:
Outflowrate = vA
In this assignment you do not have to know these formulae. But it is useful to know that both water level and drain opening size have an effect on the rate with which water flows out of the tank.