Problem $7$

A household refrigerator that has a power input of $500W$ and a COP of $1\mathrm{.}4$ is to cool $4$ large watermelons, $12$

$kg$ each, to $7C.$ If the watermelons are initially at $30C,$ determine how long it will take for the refrigerator

to cool them. The watermelons can be treated as water whose specific heat is $4\mathrm{.}2k\frac{J}{k}gC.$ Is your answer

realistic or optimistic? Explain.