Answer
$P=75\text{ psi}$
Work Step by Step
As we know that pressure $P$ and depth $h$ are directly proportional so
$P=kh$ (where k is constant of proportionality)
We plug in the known values to obtain:
$50\text{ psi}=k(10\text{ ft})$
$\implies k=\frac{50\text{ psi}}{10\text{ ft}}=5\frac{\text{ psi}}{\text{ft}}$
Thus,
$P=\left(5\frac{\text{ psi}}{\text{ ft}}\right)h$
Now we can find the pressure for a depth h=15ft as follows
$P=\left(5\frac{\text{ psi}}{\text{ ft}}\right)(15ft)=75\text{ psi}$