An offshore oil well is leaking oil onto the ocean surface, forming a circular oil slick about .005 meter thick. If the radius of the slick is r meters, the volume of oil spilled is V = .005πr² cubic meters. If the oil is leaking at a constant rate of 20 cubic meters per hour so that dV/dt = 20, find the rate at which the radius of the oil slick is increasing at a time when the radius is 50 meters.