We know that the volume of a sphere of radius r is 34πr3.
Given the radius of the spherical iron ball is R=10cm and the thickness of the ice is xcm.

Volume of ice: V=34π(R+x)3−34πR3
⇒V=34π(10+x)3−34π(10)3
Differentiating with respect to t, we get
dtdV=4π(10+x)2dtdx−0
Given dtdV=50cm3/min
⇒50=4π(10+x)2dtdx
⇒dtdx=4π(10+x)250
⇒dtdx∣x=5=4π(15)250=18π1cm/min.