Answer
$40\ lb$
Work Step by Step
Step 1. Assume $w\ (lb)$ is the weight of an object.
Step 2. Assume $d\ (mi)$ is its distance from the center of Earth.
Step 3. From the given conditions, we have $w=k/d^2$ and $90=k/(8000)^2$, thus $k=90(8000)^2$
Step 4. For $d=12,000\ (mi)$, we have $w=90(8000)^2/(12000)^2=40\ lb$
