A student is investigating how the radiation from a radioactive source changes with time. The table below shows the results from a radiation detector at intervals equal to the half-life of the source, which is exactly 5 minutes5\text{ minutes}5 minutes:
| Time (min\text{min}min) | Measured count-rate (counts per minute) |
|---|---|
| 0 | 615 |
| 5 | 315 |
| 10 | 165 |
| 15 | 90 |
Determine the constant count-rate due to background radiation, in counts per 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.