# Is t=30+5h a function?

This equation defines $t$ as a function of $h$.
For each value of $h$ it determines exactly one value of $t$, so yes it is a function.
$h$ is also a function of $t$ determined by this equation, since for every value of $t$, there is a unique value of $h$ that satisfies the equation, given by the formula:
$h = \frac{t - 30}{5}$