Consider a solid sphere of anthracite starting at $20C$ in forced water at $90C$

$.$ The diameter of the sphere is $6$ cm and its thermophysical properties can be

assumed constant as thermal conductivity $k=0\mathrm{.}26\frac{W}{mK},$ density $=1350\frac{kg}{m^3}$

and specific heat $c_p=1260\frac{J}{kgK}.$ The convection coefficient of the fluid is

$h=1000\frac{W}{m^2(K)}.$

Question $1$: Calculate the surface temperature at $20$ min $.$