From Science in the Sunshine:
Teach your students how to figure out half lives easily and effectively! After many attempts at trying to get students to understand half lives, I came up with this system of teaching them how to figure out the fractions and ratios by both having them draw the graphics and multiplying fractions.
Once a student knows how to create, and recreate these tables, they become much more adept at answering a variety of half life problems in both Earth Science and in Chemistry.
The worksheets are set up so that they can be used with a wide range of abilities, from an answer key that has everything written down already for them, to one partially filled in, to a completely blank graphic that they have to fill in themselves.
In addition, there is a handout that explains the definition of half lives as well a brief explanation of one theory of how some half lives occur. This is good for background information, and can also be used for the generation of a quiz question or two.