A detector of ionising radiation gives a background reading of 15 counts/second15\text{ counts/second}15 counts/second.
A radioactive isotope with a half-life of 4.0 hours4.0\text{ hours}4.0 hours is brought near to the detector. The reading on the detector increases to 135 counts/second135\text{ counts/second}135 counts/second.
How long does it take for the reading on the detector to decrease to 30 counts/second30\text{ counts/second}30 counts/second?
4.0 hours4.0\text{ hours}4.0 hours
8.0 hours8.0\text{ hours}8.0 hours
12 hours12\text{ hours}12 hours
16 hours16\text{ hours}16 hours
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.