$5.$ Consider a brick house $($k $=0\mathrm{.}72$ W$/$m C and $=1\mathrm{.}9$ x $10-5$ m $2$

$/$s$)$ whose walls are $10$ m long, $3$m high, and $0\mathrm{.}3$ m thick. The heater of the house broke down one night, and the entire house,including its walls, was observed to be $5$ C throughout in the morning. The outdoors warmed upas the day progressed, but no change was felt in the house, which was tightly sealed. Assumingthe outer surface temperature of the house to remain constant at $18$ C$,$ determine how long itwould take for the temperature of the inner surfaces of the walls to rise to $7\mathrm{.}6$ C$.$

Answer: $24$ min