*An example question:*

*The activity of a radioisotope is 640 cpm (counts per minute). Two hours later, it has fallen to 80 cpm. Find the half life of this sample.*

**Answer**

So we start of with 640 cpm

A half life of this would be 320 as we DIVIDE it by 2 (HALF-life)

We then half it again to equal 160. This is the 2nd half-life.

We then half it again to equal 80. This is the 3rd half-life.

Two hours is the same as 120 minutes (I find it easier to work in minutes)

Because we worked out it took 3 half-lives to get to 80cpm, we divide the two hours (120 minutes) by 3. The answer to this question is 40 minutes!

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