$3-$Layers of $23-$cm$-$thick meat slabs $-$q cmadca$=0\mathrm{.}13{10}^{-6}\frac{m^2}{s})$ initially at a uniform temperatures of $7$ C a eesto be frozen by refrigerated air at $-30C$ flowing at a velacity of $1\mathrm{.}4\frac{m}{s}.$ The average heat transfer coefficient between the meat and the air is $20\frac{W}{m^{2.\text{@}}}C.$ Assuming the size of the meat slabs to be large relative to their thickness, determine how long it will take for the center temperature of the slabs to drop to $18C.$ Also, determine the surface temperature of the meat slab at that time.