GRADE 7 SCIENCE · CHAPTER 1
Engineering & design
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Engineering design
Measure a model filter’s particle reduction
Use before-and-after counts to assess one specified function of a model filter.
AN EXAMPLE FROM THIS CHAPTER
A model filter is tested with harmless visible particles. An equal-volume sample has 100 counted particles before filtering and 20 after. What percentage reduction is shown?
Show answer & explanation
80%
The reduction is 100 − 20 = 80 particles. Divide by the starting count and multiply by 100: × 100 = 80%. This tests counted particles, not all possible water contaminants.
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