Water is draining from an inverted conical tank whose point is at the bottom. The tank is 10 ft deep and 4 ft across at its widest point. At the instant when the water is 5 ft deep, the volume of water is decreasing at 2 ft³/min. Calculate the rate, in ft/min, at which the radius of the water’s surface is changing at that instant. Indicate the sign of your answer.