Question

Half-life: The activity of a sample is initially 80 decays per min. It becomes 5 decays per min after 4h. What is the half life?

Answer 1

I'm not sure how to convert decays into the amount remaining.

Answer 2

Check out the Nuclear Decay Probability section (it's the 2nd one):

http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/halfli2.html

And look at the last equation they show in that section :)

Answer 3

And to help you out more, lambda there is related to the half-life through a very simple equation written somewhere in the first section of that same page :)

Answer 4

thanks :)

Answer 5

For any quick physics references and brush ups, that website is super handy!