Grade 7 · Science
Measure a model filter’s particle reduction
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A model filter is tested with harmless visible particles. An equal-volume sample has 200 counted particles before filtering and 50 after. What percentage reduction is shown?
- 37.5%
- 75%
- 80%
- 70%
See the answer & why
75%
The reduction is 200 − 50 = 150 particles. Divide by the starting count and multiply by 100: × 100 = 75%. This tests counted particles, not all possible water contaminants.
A little hint: Percentage reduction = (starting count − final count) ÷ starting count × 100.
State the measured goal: reducing a specified kind of harmless visible particle in equal-volume samples.
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