$)$ Dates at $20$oC are compressed into a $20$mm thick slab and then placed on a rack in a cold room at $-$

$7$oC$.$ Assume air cools both sides of the slab equally $($and not the edges$).$ Calculate the time to cool

the centre of the slab to $-4$oC$.$ See date$/$air properties below:

Product thermal conductivity $($k$)0\mathrm{.}337$ W$.$m$-1.$oC$-1$

Product density $($r$)1319$ kg$.$m$-3$

Product thermal diffusivity $($a$)1\mathrm{.}00$E$-07$ m$2.$s$-1$

Convection heat transfer coefficient $($h$)13$ W$.$m$-2.$oC$-1$