Oil spilled from a ruptured tanker spreads in a circle whose area increases at a constant rate of 4 mi²/hr. How rapidly is the radius of the spill increasing when the area is 10 mi²?
Hints: 1) How can you relate the rate of change in area to the rate of change in radius?
2) What is the radius when the area is 10 mi^2? This is the value of the radius you need to substitute when you substitute area = 10 mi^2.