Liquid flows out of a tank at a rate given by Toricelli's law,

, where

* *is the volume and

the height of the water in the tank (both functions of time),

is the radius of the tank,

* *is the radius of the hole in the bottom of the tank,

= 9.81

, the acceleration due to gravity, and

is the time. The volume is given by

and the solution of the differential equation gives

. Thus the length of time required to drain the tank, so that

, is given by

seconds.