A radioactive source has a half-life of 20 minutes20\text{ minutes}20 minutes. A detector near the source shows a reading of 8000 counts per second8000\text{ counts per second}8000 counts per second. Background radiation can be ignored. What is the reading on the detector 80 minutes80\text{ minutes}80 minutes later?
500 counts per second500\text{ counts per second}500 counts per second
1000 counts per second1000\text{ counts per second}1000 counts per second
2000 counts per second2000\text{ counts per second}2000 counts per second
4000 counts per second4000\text{ counts per second}4000 counts per second
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.