#### Answer

$s\approx 46\ $mi/h

#### Work Step by Step

Since $D$ varies directly as the square of the speed $s$, we have:
$D=ks^{2}$
We are given: $D=150$ and $s=40$, so we have:
$150=k(40)^{2}$
$k=0.09375$
Therefore:
$D=0.09375s^{2}$
If we plug in $D=200$, we can find $s$:
$200=0.09375s^{2}$
$s=\sqrt{200/0.09375}$
$s\approx 46\ $mi/h