Matilda has a 100 g rock sample. she knows that the rock contains uranium-238 and that the half-life of uranium-238 is 4.5 x 109 years. if she cuts the rock sample in half, what happens to the half-life of the uranium? the half-life increases. the half-life decreases. the half-life remains the same.

Look at the scientific fact right there in the question:

"... the half-life of uranium-238 is 4.5 X 10⁹ years ."

Does that say anything about the size of the sample ?  No !  It says nothing about the size of the sample !  The half-life is the time it takes for half of however much you start with to decay.  

The length of the half life only depends on what substance you have, NOT on how much of it you have.

It could be a microscopic dot of it, or a supertanker full of it.  those two samples both have the same half-life.

After Matilda has cut her rock in half, the half life of each piece remains the same as the half life of the whole rock was before she cut it.

the answer is 100. i just did this lesson and got a 100% on it. : )

-fishtank