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Grade 7 · Science

Measure a model filter’s particle reduction

Use before-and-after counts to assess one specified function of a model filter.

6 authored questions + worked example

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Worked example

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?

Answer & why

Answer: 80%

The reduction is 100 − 20 = 80 particles. Divide by the starting count and multiply by 100: 80/100 × 100 = 80%. This tests counted particles, not all possible water contaminants.

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All questions

All 6 authored questions for this skill.

Question 1

A model filter is tested with harmless visible particles. An equal-volume sample has 60 counted particles before filtering and 30 after. What percentage reduction is shown?

  1. 55%
  2. 50%
  3. 45%
  4. 25%
A hint

Percentage reduction = (starting count − final count) ÷ starting count × 100.

Answer & why

Answer: 50%

The reduction is 60 − 30 = 30 particles. Divide by the starting count and multiply by 100: 30/60 × 100 = 50%. This tests counted particles, not all possible water contaminants.

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Question 2

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?

  1. 37.5%
  2. 75%
  3. 80%
  4. 70%
A hint

Percentage reduction = (starting count − final count) ÷ starting count × 100.

Answer & why

Answer: 75%

The reduction is 200 − 50 = 150 particles. Divide by the starting count and multiply by 100: 150/200 × 100 = 75%. This tests counted particles, not all possible water contaminants.

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Question 3

A model filter is tested with harmless visible particles. An equal-volume sample has 120 counted particles before filtering and 30 after. What percentage reduction is shown?

  1. 70%
  2. 75%
  3. 37.5%
  4. 80%
A hint

Percentage reduction = (starting count − final count) ÷ starting count × 100.

Answer & why

Answer: 75%

The reduction is 120 − 30 = 90 particles. Divide by the starting count and multiply by 100: 90/120 × 100 = 75%. This tests counted particles, not all possible water contaminants.

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Question 4

A model filter is tested with harmless visible particles. An equal-volume sample has 50 counted particles before filtering and 5 after. What percentage reduction is shown?

  1. 45%
  2. 85%
  3. 95%
  4. 90%
A hint

Percentage reduction = (starting count − final count) ÷ starting count × 100.

Answer & why

Answer: 90%

The reduction is 50 − 5 = 45 particles. Divide by the starting count and multiply by 100: 45/50 × 100 = 90%. This tests counted particles, not all possible water contaminants.

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Question 5

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?

  1. 40%
  2. 85%
  3. 80%
  4. 75%
A hint

Percentage reduction = (starting count − final count) ÷ starting count × 100.

Answer & why

Answer: 80%

The reduction is 100 − 20 = 80 particles. Divide by the starting count and multiply by 100: 80/100 × 100 = 80%. This tests counted particles, not all possible water contaminants.

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Question 6

A model filter is tested with harmless visible particles. An equal-volume sample has 80 counted particles before filtering and 20 after. What percentage reduction is shown?

  1. 37.5%
  2. 70%
  3. 75%
  4. 80%
A hint

Percentage reduction = (starting count − final count) ÷ starting count × 100.

Answer & why

Answer: 75%

The reduction is 80 − 20 = 60 particles. Divide by the starting count and multiply by 100: 60/80 × 100 = 75%. This tests counted particles, not all possible water contaminants.

Add to plan