A spherical iron ball of radius 10 cm is coated with a layer of ice of uniform thickness that melts at a rate of $50 \mathrm{~cm}^3 / \mathrm{min}$. When the thickness of the ice is 5 cm , then the rate at which the thickness (in $\mathrm{cm} / \mathrm{min}$ ) of the ice decreases, is :
