A water tank has the shape of an inverted right circular cone, whose semi-vertical angle is ${\tan ^{ - 1}}\left( {{1 \over 2}} \right)$. Water is poured into it at a constant rate of 5 cubic meter per minute. The the rate (in m/min.), at which the level of water is rising at the instant when the depth of water in the tank is 10m; is :-