A cylindrical tank with length of 5 ft. and radius 3 ft. is situated with its axis horizontal. If a circular bottom hole with a radius of 1 in. is opened and the tank is initially half full of xylene, how long will it take for the liquid to drain completely?

this is an application of Toricelli's principle ... the speed at which a fluid exits a hole is a function of the height of the fluid above the opening.

further, the rate of change of volume w/r to time has the relationship

 , where   is the area of the opening.

let   be at the tank's center. the volume of fluid in the tank as a function of the fluid's height above the opening ...

at  ,   ... 

when   ...

 or a little over 6 min.