Annular aluminum fins of rectangular profile are attached to a circular tube having an outside diameter of 50 mm and an outer surface temperature of 250 °C. The fins are 4 mm thick and 15 mm long. There are 120 fins per meter of tube length. The system is in ambient air at a temperature of 25 °C, and the surface convection coefficient is 50 W/(m²·K). a) What is the efficiency of a single fin? (10 pts) b) What is the effectiveness of a single fin? (10 pts) c) What is the overall surface efficiency? (15 pts) d) What is the rate of heat transfer per unit length of the tube? (15 pts)