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