Answer
$d$=dependent variable
$t$=independent variable
Domain: [0,8]
Work Step by Step
Since the distance depends on the time that the stone spends in the air, $d$ is the dependent variable and $t$ is the independent variable. The domain consists of the appropriate values of $t$. The stone's path begins at $t=0$ and ends when it hits the ground. We find the time that it hits the ground by setting $d=0$:
$40t-5t^2=5t(8-t)0$
$t=0$, $t=8$
Thus the domain is: [0,8]
