An environmental protection agency monitors heavy metal contamination in river water samples. The table below shows the results of water quality assays conducted in a river basin during two distinct monitoring campaigns in 2018 and 2023.
| Monitoring Year | Number of water samples analyzed | Percentage of samples exceeding the safe cadmium threshold (%) |
|---|---|---|
| 2018 | 12,400 | 3.5 |
| 2023 | 35,500 | 0.8 |
Calculate the decrease in the number of water samples exceeding the safe cadmium threshold between 2018 and 2023.
45 exam-style questions on OCR A A Level Biology Analysis. Each one has a worked solution and a mark scheme showing where the marks go.