A detector placed near a radioactive isotope gives a measured count rate of 1000 counts/minute1000\text{ counts/minute}1000 counts/minute. The half-life of the isotope is five hours.
The background count rate is 40 counts/minute40\text{ counts/minute}40 counts/minute.
What is the count rate on the detector after fifteen hours?
120 counts/minute120\text{ counts/minute}120 counts/minute
125 counts/minute125\text{ counts/minute}125 counts/minute
160 counts/minute160\text{ counts/minute}160 counts/minute
165 counts/minute165\text{ counts/minute}165 counts/minute
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.