Find the heat flux across a composite slab of width $0\mathrm{.}25$ m and $0\mathrm{.}15$ m of conductivity $388$ W$/$m$-$K and $250$ W$/$m$-$K respectively when its one surface is at $150\backslash$deg C and the other surface is at $40\backslash$deg C$.$ Also find the temperature at the joint of two materials of the slab.