A copper slab $(\{$:$k=401\frac{W}{m}*K,a=117{10}^{-6}\frac{m^2}{s})$ of thickness $2cm$ is initially

at a temperature of $25C.$ At $t=0,$ one side of the slab starts receiving a net heat flux of

$5000\frac{W}{m^2},$ while the other side dissipates heat to the surrounding fluid at a temperature of

$25.C$ with an average heat transfer coefficient of $70\frac{W}{m}2*K.$ How long does it take for the slab temperature to reach

$60*C?$