Answer
$75psi$
Work Step by Step
Because pressure varies directly as depth, the relationship can be written in form of: $P=kx$, where P is pressure and x is the depth.
With given information, we are able to find $k$.
$50=k*10$
$k=50/10=5 (psi/ft)$
Now we can calculate pressure at the depth of $15ft$.
$P=5*15=75 (psi)$
