Field Notesgrade 7 math
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Solution key: Parts of a hundred

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Camp · Parts of a hundred

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1 of 10 · percent-change-c01

20% of 90?

  1. We want 20% of 90. Percent means out of 100.
  2. 20% = 20/100 = 0.2.
  1. “Of” means multiply the whole by that fraction: 90 × 0.2 = 18.
  2. So 20% of 90 is 18. This is smaller than 90, because 20% is less than 100%.

Answer: 18

2 of 10 · percent-change-c02

15% of 80?

  1. We want 15% of 80. Percent means out of 100.
  2. 15% = 15/100 = 0.15.
  1. “Of” means multiply the whole by that fraction: 80 × 0.15 = 12.
  2. So 15% of 80 is 12. This is smaller than 80, because 15% is less than 100%.

Answer: 12

3 of 10 · percent-change-c03

A $50 shirt is 10% off. Sale price?

  1. Start with $50. A 10% decrease subtracts from this ORIGINAL amount.
  2. 10% = 0.1. Find the change: 50 × 0.1 = 5.
  1. Subtract the change: 50 − 5 = 45.
  2. The new amount is $45, less than the original 50.

Answer: 45

4 of 10 · percent-change-c04

A $40 bill becomes 25% larger. New amount?

  1. Start with $40. A 25% increase adds to this ORIGINAL amount.
  2. 25% = 0.25. Find the change: 40 × 0.25 = 10.
  1. Add the change: 40 + 10 = 50.
  2. The new amount is $50, greater than the original 40.

Answer: 50

5 of 10 · percent-change-c05

What percent of 40 is 10?

  1. We know the part, 10, and the whole, 40. We want the part as a percent of that whole.
  2. Divide part by whole: 10 ÷ 40 = 1/4 = 0.25.
  1. Convert the decimal to percent: 0.25 × 100 = 25%.
  2. The answer is 25%. Check: 0.25 × 40 = 10.

Answer: 25 %

6 of 10 · percent-change-c06

Decrease 80 by 5%. New value?

  1. Start with 80. A 5% decrease subtracts from this ORIGINAL amount.
  2. 5% = 0.05. Find the change: 80 × 0.05 = 4.
  1. Subtract the change: 80 − 4 = 76.
  2. The new amount is 76, less than the original 80.

Answer: 76

7 of 10 · percent-change-c07

Increase 200 by 12%. New value?

  1. Start with 200. A 12% increase adds to this ORIGINAL amount.
  2. 12% = 0.12. Find the change: 200 × 0.12 = 24.
  1. Add the change: 200 + 24 = 224.
  2. The new amount is 224, greater than the original 200.

Answer: 224

8 of 10 · percent-change-c08

30% of 250?

  1. We want 30% of 250. Percent means out of 100.
  2. 30% = 30/100 = 0.3.
  1. “Of” means multiply the whole by that fraction: 250 × 0.3 = 75.
  2. So 30% of 250 is 75. This is smaller than 250, because 30% is less than 100%.

Answer: 75

9 of 10 · percent-change-c09

A price drops from 80 to 60. Percent decrease?

  1. Percent change compares the change with the ORIGINAL 80, not the new 60.
  2. Find the decrease: 80 − 60 = 20.
  3. Divide by the original: 20 ÷ 80 = 0.25.
  1. Multiply the decimal by 100: 0.25 × 100 = 25%.
  2. The decrease is 25%. Check: 80 × 0.25 = 20, the change.

Answer: 25 %

10 of 10 · percent-change-c10

5% of 60?

  1. We want 5% of 60. Percent means out of 100.
  2. 5% = 5/100 = 0.05.
  1. “Of” means multiply the whole by that fraction: 60 × 0.05 = 3.
  2. So 5% of 60 is 3. This is smaller than 60, because 5% is less than 100%.

Answer: 3