Answer
15 pounds
Work Step by Step
weight = k/distance^2
160 pounds, 4000 miles from center
$160 = k/4000^2$
$160=k/16000000$
$160*16000000=k/16000000*16000000$
$2560000000 = k$
weight unknown, 4200 miles from center
$w=k/4200^2$
$w=2560000000/17640000$
$w=256000/1764$
$w=14.51