$8.$ A fin of rectangular cross$-$section with a thickness of $5$ mm$,$ a height of $50$ mm$,$ and a length of $100$ mm is attached to a base with a thermal conductivity of $100$ W$/$mK$.$ The base temperature is $100\backslash$deg C$,$ and the air temperature is $20\backslash$deg C$.$ Determine the heat transfer rate from the fin and the fin efficiency.