The sand falling from a hopper at `10*pi` ft^3/s forms a conical pile where the radius is always equal to the height.

The volume of a cone if the radius r is also equal to the height is given by `V = (1/3)*(pi*r^2)*r = (1/3)*pi*r^3`

`(dV)/(dr) = pi*r^2`

`(dV)/(dt) = 10*pi`

`((dV)/(dt))/((dr)/(dt)) = pi*r^2`

=> `(10*pi)/(pi*r^2) = (dr)/(dt)`

=> `(dr)/(dt) = 10/(r^2)`

At r = 5

`(dr)/(dt) = 10/25 = 2/5` ft/s

The radius is increasing at `2/5` ft/s