Question 2:
    Another substance has a half-life of 3 hours.
      What fraction of the atoms will have decayed after 6 hours?

Answer:

Three-quarters of the atoms will have decayed after 6 hours.

The half-life is 3 hours, so 6 hours is 2 half-lives. After 2 half-lives only one quarter of the original atoms remain, which means that three-quarters of them have decayed.