Question

The force of gravity between two masses was 18 N when they were 13 meters apart. The distance between them was changed and the force became 76 N. How far apart were they then?

Answer 1

F = G*M1M2/d^2
Since M1 and M1 and G are constant this becomes
F = k/d^2
18 = k/13^2 so k = 3042
Then 76 = 3042/d^2 and solve for d = about 6
One of you physics/math/science people should check this out.

Answer 2

Looks good, but you don't really need to know k, since you know that Fd^2 is constant. You want d such that
76d^2 = 18*13^2
d = 6.32
as you showed above

Answer 3

Thank you.