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