#### Answer

1.581 seconds

#### Work Step by Step

$h=-16t^2+c$
dropped from height of 40 feet to hit the ground
$h=0$
$c=40$
$0=-16t^2+40$
$16t^2=40$
$16t^2/16=40/16$
$t^2=5/2$
$\sqrt {t^2} = \sqrt {5/2}$
$t=1.581$
Usually, we would have -1.581 as another answer. However, we cannot have a negative time. (Thus, -1.581 is an extraneous solution.)