Answer
$$M=8z$$
Work Step by Step
According to the information given, we can write the following relation:
$M=kz$
Where $k$ is constant of proportionality.
if $M=120$ and $z=15$, we can find $k$ and write general equation of this variation.
$120=15k$
$k=\frac{120}{15}$
$k=8$
$M=8z$
