Tuesday
Tuesday: Percent of a Number, and the Whole
Raven draws a double number line at the Numbers Desk. "Percent on top, kids underneath. 100% over 40."
Rocket fills in the middle. "50% over 20. 25% over 10. 10% over 4."
"Easy," he says. "What is 75% of 40? Three 25s. Thirty kids."
Then Raven flips it. "Twelve kids want to help at the relay. That is 30% of the helpers. How many helpers?"
Rocket freezes. "The whole is missing. I only have the part."
Nova hovers beside the line. "What do you notice about 30% and 12? What would 10% be?"
"Four," Rocket says. "So 100% is ten fours. Forty helpers." He writes it down, slowly this time.
The method: percent of a number
- Write the percent as a fraction over 100: 30% = 30/100.
- Multiply by the whole: 30/100 × 40 = 12.
- Or find 10% first by dividing by 10, then multiply: 10% of 40 is 4, so 30% is 12.
- Check with a benchmark: 30% is a bit less than one third, and one third of 40 is about 13.
| Benchmark percent | As a fraction | As a decimal | Of 40 kids |
|---|---|---|---|
| 10% | 1/10 | 0.1 | 4 |
| 25% | 1/4 | 0.25 | 10 |
| 50% | 1/2 | 0.5 | 20 |
| 75% | 3/4 | 0.75 | 30 |
| 100% | 1 | 1.0 | 40 |
The method: find the whole
Sometimes you know the part and the percent, but not the whole. Twelve helpers are 30% of all the helpers.
Find 1% first: 12 ÷ 30 = 0.4 helper per percent. Then 100% is 0.4 × 100 = 40 helpers.
Or use 10%: if 30% is 12, then 10% is 4, so 100% is 40. Both ways land on the same whole.
- Read the question.
- Tap your answer.
- Read the question.
- Tap your answer.
| Number sentence | True or false? |
|---|---|
| 25/100 = 0.25 | ? |
| 1/2 = 0.5 | ? |
| 10/100 = 0.01 | ? |
| 3/4 = 0.75 | ? |
| 30/100 × 40 = 12 | ? |
- ?1/5
- ?40%
- ?0.3
- ?15%
Two methods down. Tomorrow the Numbers Lab turns counters into percents.