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Comparing bacterial population values
Sewage was accidentally added to a river. The sewage moved with the river water and affected the number of bacteria in the water and the concentration of dissolved oxygen in the water. Samples of river water were analysed at different distances from where the sewage was added. Table 2 shows the results.
| Distance from where sewage was added in km | Number of bacteria × 1000/mm³ | Concentration of dissolved oxygen in mg/dm³ |
|---|---|---|
| 0.0 | 4 | 5.4 |
| 1.0 | 75 | 4.8 |
| 2.0 | 125 | 4.3 |
| 3.0 | 145 | 3.8 |
| 4.0 | 160 | 3.3 |
| 5.0 | 216 | 2.7 |
The number of bacteria at 5.0 km was greater than the number of bacteria where the sewage was added. Calculate how many times greater.
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Marking points
- 1 Divides the bacteria value at 5.0 km by the value at 0.0 km and obtains 54.
Full-mark answer
216 ÷ 4 = 54 times greater.
Why this answer loses marks
I subtracted the two readings.
Subtraction finds an absolute difference; “times greater” requires the larger comparison value divided by the baseline.