Answer
$t=4$ hours
Work Step by Step
If $s(t)=1500$
$ 1500=475t-25t^{2}\qquad$ divide with 25
$60=19t-t^{2}$
$t^{2}-19t+60=0$
$(t-15)(t-4)=0$
Of the two possible solutions, we discard $t=15$,
and take $t=4$ hours as a viable solution.
We explain why in part (c).
