4. A thin layer of water 0.01 ft thick in a flat pan of 1.0 ft is held at a constant temperature of 68 F and total pressure of dry air above the pan oil atm and and at 68 F. Assuming the evaporation to take place by molecular diffusion through a gas film of constant thickness of 0.05 ft, calculate the time in hours for the water to evaporate completely.(20 points) DAB= 0.248 cm/sec