Answer
$(a)$ $C=kpm$
$(b)$ $k=\frac{1}{8}$
$(c)$ $C= \$57,500$
Work Step by Step
$(a)$ According to the problem, we have $C=kpm$
Where $k$ is the constant of proportionality.
$(b)$
$C=\$60,000$
$p=120$
$m=4000$
$k$ is unknown
$60000=k\times120\times4000$
$480000k=60000$
$k=\frac{1}{8}$
$(c)$
$m=5000$
$p=92$
$C$ is unknown
$C=\frac{1}{8}\times 5000\times 92 = \$57,500$