You may be asked to use information on half-life to work out the decline in count rate after a given number of half-lives. Here's a typical question. A radioactive isotope has a half-life of 15 days and an initial count rate of 200 counts per second. Determine the count rate after 45 days. After each half-life the count rate will have halved. The half-life is 15 days so 45 days.