Question

If a original sample of radioactive uranium-238 had a mass of 400 grams,what is the total amount of the uranium-238 sample that would be left after 9 billion years?

Answer 1

Since the half-life is 4.5 billion years, two half-lives would have elapsed after 9 billion years. 
If you start with 400 g, half decays during the first half-life, leaving 200 g. 
During the second half-life, half of the 200 g decays, leaving 100 g.