The total count rate measured from a radioactive source (which includes a constant background radiation) is recorded over time. At t=0t = 0t=0, the measured total count rate is 250 counts/minute250\text{ counts/minute}250 counts/minute. The constant background radiation is 30 counts/minute30\text{ counts/minute}30 counts/minute.
What is the corrected count rate of the source after a time period equal to three half-lives of the radioactive isotope?
144 exam-style questions on CIE IGCSE Physics 5.2.4a Half-life. Each one has a worked solution and a mark scheme showing where the marks go.