A household refrigerator that has a power input of 450 W and a COP of 2.5 is to
cool five large watermelons, 10 kg each, to 8°C. If the watermelons are initially at 20°C,
determine how long it will take for the refrigerator to cool them. The watermelons can
be treated as water whose specific heat is 4.2 kJ/kg · °C. Is your answer realistic or
optimistic? Explain.