A person coughs when a foreign object is in the windpipe. The velocity of the cough depends on the size of the object. Suppose a person has a windpipe with a 20-mm radius. If a foreign object has a radius r, in millimeters, then the coughing velocity V(r), in millimeters per second, needed to remove the object is given by V(r)= k(20r^2 - r^3), 0 <= r <= 20

where k is some positive constant. For what size object is the maximum velocity required to remove the object?