#### Answer

5 seconds

#### Work Step by Step

Substitute the given distance and solve for t.
$d=16t^2$
Substitute the distance for d.
$400=16t^2$
Divide both sides by 16.
$400 \div 16=16t^2\div16$
Simplify.
$25=t^2$
Take the square root of each side.
$\sqrt{25}=\sqrt{t^2}$
Simplify.
$5=t$