Answer
$\approx 124.76$ lb.
Work Step by Step
If the weight $w$ varies inversely with $d^{2}$, then
there is a nonzero constant $k$ such that
$w=\displaystyle \frac{k}{d^{2}}$
---
If $ w=125$ when $d=3960$,
substituting, we find k:
$125=\displaystyle \frac{k}{3960^{2}}\quad/\times 3960$
$k=1.9602\times 10^{9}$
Thus, we write: $\displaystyle \quad w=\frac{1.9602\times 10^{9}}{d^{2}}.$
If $d=3963.8$,
$w=\displaystyle \frac{1.9602\times 10^{9}}{(3963.8)^{2}}\approx 124.76$ lb.
