A 20 by 20 cm slab of copper 5 cm thick at a uniform temperature of 260C

Question:

A 20 by 20 cm slab of copper 5 cm thick at a uniform temperature of 260ºC suddenly has its surface temperature lowered to 35ºC. Using the concepts of thermal resistance and capacitance and the lumped-capacity analysis, find the time at which the center temperature becomes 90ºC; ρ = 8900 kg/m3, cp = 0.38 kJ/kg·ºC, and k = 370 W/m · ºC.