Answer
Domain = $\{t\in \mathbb{R}\ \ |\ 0\leq t\leq 60\}$
Work Step by Step
If a function is used as a model for an application, the problem situation may require restrictions on the domain. For example,
length and time are generally nonnegative, and a person's age does not increase indefinitely.
$t$ is a nonnegative value, and so is $r(t),$ so we restrict t with
$ 900-15t\geq 0\qquad$... add 15t
$ 900\geq 15t\qquad$... divide with 15
$60\geq t$
Domain = $\{t\in \mathbb{R}\ \ |\ 0\leq t\leq 60\}$
