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

