Answer
$5.477$ miles out
Work Step by Step
The first thing we should do is convert the height of the conning tower into miles
$\frac{20ft}{5280ft/mi} \approx .0037879 mi$
Based on the picture, we can see that
$d^2=(3960+.0037879)^2-3960^2=30$
so $d\approx 5.477$ miles.
