Answer
$48 m/s$
Work Step by Step
The fuction of accleleration is the derivative of the function of velocity.
$a(t)=v'(t)$
Given $v'(t)=1.6\quad $
(a constant value, for which we showed in exercise 32 that $v(t)$ is linear), it follows that
$v(t)=1.6t+C$
At time $t=0$, we are holding the rock, ready to let it fall. Its velocity at t=0 is zero.
$0=1.6(0)+C\quad \Rightarrow\quad C=0\quad \Rightarrow v(t)=1.6t$
After t=30 seconds,
$v(30)=1.6(30)=48 m/s$
