A detector is placed near a radioactive isotope and records a count rate of 840 counts/min840\text{ counts/min}840 counts/min.
The half-life of the isotope is 10 min.
The average background count rate is 40 counts/min40\text{ counts/min}40 counts/min.
What is the count rate measured by the detector after 30 min?
100 counts/min100\text{ counts/min}100 counts/min
105 counts/min105\text{ counts/min}105 counts/min
140 counts/min140\text{ counts/min}140 counts/min
240 counts/min240\text{ counts/min}240 counts/min
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.