If a rock initially contained 10 milligrams of a radioactive substance when it first formed, how much remains after 4 half-lives?

Well after the first half life it has 10/2 5 milligrams and the after the second half life it has 5/2 2.5
And the third 2.5/2 is 1.25
And the fourth 1.25/2 is 0.625

Maybe you should get some from your teacher and learn more so you can get your grades up and maybe study a bit more

The answer is falsegravitational force is independent of medium and depends only on the masses of the two objects and the distance between them.