Answer
$\frac{640}{9}\approx71.1\ kg$
Work Step by Step
Step 1. Since $w$ varies inversely as the square of $d$, we can write $w=\frac{c}{d^2}$ where $c$ is a constant.
Step 2. As $w=90\ kg$ when $d=6400\ km$, we have $90=\frac{c}{6400^2}$, thus $c=90\cdot 6400^2$
Step 3. For $d=6400+800=7200\ km$, we have $w=\frac{90\cdot 6400^2}{7200^2}=\frac{640}{9}\approx71.1\ kg$
