Answer
$256$ feet
Work Step by Step
Let $s$ be the speed of an anvil.
Let $d$ be the distance of an anvil's fall.
$s=k\sqrt d$
$s=160; d=400$
$160=k\sqrt {400}$
$160=k(20)$
$k=8$
$s=8\sqrt d$
$s=128$
$128=8\sqrt d$
$\sqrt d=16$
$d=256$ feet