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.

hope I didn't commit any egregious errors ... I'm sure someone will let us both know if I did.