Question 1:
          A radioactive substance has a half-life 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 half-life of the substance is 30 minutes. 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 remain.

 

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Created by Andy Darvill, www.darvill.clara.net,Science teacher at Broadoak Community School, Weston-super-Mare, England