Answer
The person can see about $12.25$ miles out
Work Step by Step
The first thing we should do is convert the height of the conning tower into miles
$\frac{100ft}{5280ft/mi}≈.0189mi$
Based on the picture, we can see that
$d^2=(3960+.0189)^2−3960^2=150$
so $d\approx 12.25$ miles.
