Answer
$d$=dependent variable
$t$=independent variable
Domain: [0,2]
Work Step by Step
Since the distance depends on the time elapsed, distance $d$ is the dependent variable and time $t$ is the independent variable. The domain consists of the appropriate values of $t$. The time begins with $t=0$ and ends when the stone hits the river. We can find this time by setting $d=0$:
$20-5t^2=5(4-t^2)=5(2-t)(2+t)=0$
$t=2$, $t=-2$
We ignore $t=-2$, since this solution goes backwards in time.
Thus the domain is [0,2]
