Walls of a cubical oven are of thickness l$,$ and they are made of material of thermal conductivity k$.$ The temperature inside the oven is $100$C and the inside heat transfer coefficient is $3$k$/$l$.$ If the wall temperature on the outside is held at $25$C$,$ what is the inside wall temperature in degree centigrade?

$35\mathrm{.}5$

$43\mathrm{.}75$

$81\mathrm{.}25$

$48\mathrm{.}25$