Water is flowing from a tap at the base of a cylindrical tank at a constant rate. Ryan wants to determine the time it will take to empty the tank. Which measurement (s) should Ryan use?

As the water leaves the tank, the water level will change, so the volume of the water will change accordingly. The volume of water at any time is determined by the radius of the tank and the water level at that time. Since the tank's radius is constant, Ryan only needs to check the water level (i. e. measure the height of the surface of water from the bottom of the tank.