radioactive substance has a half-life of 30 minutes.
What fraction of
the atoms will not have decayed after 1 hour?
One quarter of the atoms will not have
decayed after 1 hour.
The half-life of the substance is 30
That means that 1 hour is 2 half-lives for the substance.
After one half-life the activity will fall
to 1/2 of the starting value, after a second half-life it
will have fallen to half of that - i.e. a quarter.
So one quarter of the original atoms