A long, cast-iron cylinder 0.2 m in diameter and initially at 95°C is cooled by contacting it with a stream of liquid water at a constant temperature of 25°C. The convective heat transfer coefficient is h = 1,200 W/m² K.

(a) Determine the time needed for the cylinder to reach 55°C if the external resistance is assumed to be dominant.

(b) Determine the time needed for the average temperature of the cylinder to reach 55°C if the internal resistance is assumed to be dominant.

(c) Determine the time needed for the average temperature of the cylinder to reach 55°C if both internal resistance and external resistances are considered.