A spherical snowball is melting. It's radius is decreasing at .4 cm per hour when the radius is R cm. How fast is it's volume decreasing at that time?
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V = (4/3) * pi * r^3
dV/dt = 4 * pi * r^2 * dr/dt
r = -0.4
r = R
dV/dt = 4 * pi * R^2 * (-0.4)
dV/dt = -1.6 * pi * R^2
dV/dt = 4 * pi * r^2 * dr/dt
r = -0.4
r = R
dV/dt = 4 * pi * R^2 * (-0.4)
dV/dt = -1.6 * pi * R^2