The height h of water that is flowing through an orifice at the bottom of a cylindrical...

The height h of water that is flowing through an orifice at the bottom of a cylindrical tank is given by

Where are the cross-sectional areas of the water and orifice, respectively (see Example 8, section 1.2). solve the equation if the initial height of the water is 20ft and At what time is the tank empty?