Mathematics K–6 · Years 5–6
Percentages on a metre rule and a jug
Number and algebra
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The idea
A percentage is a number of hundredths of a whole, so on a metre rule 40 percent is 40 cm and on a 250 mL jug it is 100 mL.
What you need
- a metre rule marked in centimetres and a strip of paper 1 m long
- a 250 mL measuring jug, a 1 L jug and water
- a set of percentage cards (10, 25, 40, 50, 75, 100) and a set of fraction and decimal cards
- shop-price cards showing a price and a discount of 10, 25 or 50 percent
How to do it
- Fold the metre strip in half, then in quarters, then in tenths using the rule. Label the folds with fractions, decimals and percentages.
- Place each percentage card at its mark on the metre rule and read the length in centimetres.
- Pour water into the 250 mL jug to 50 percent, then 25 percent, then 40 percent, reading the millilitres each time. Repeat in the 1 L jug.
- Match fraction, decimal and percentage cards into triples and check each triple on the rule.
- For each shop card, find the discount in dollars and the sale price, then check with the metre rule scaled so that 100 cm stands for the full price.
What you should see
On the metre rule 10 percent is 10 cm, 25 percent is 25 cm, 40 percent is 40 cm and 75 percent is 75 cm. In the 250 mL jug 50 percent is 125 mL, 25 percent is 62.5 mL (read as about 60 to 65 mL) and 40 percent is 100 mL; in the 1 L jug the same percentages are 500, 250 and 400 mL. The triples match one quarter with 0.25 and 25 percent, one half with 0.5 and 50 percent. A $40 item at 25 percent off has a discount of $10 and a sale price of $30.
What changes
This activity lists no variables to change, measure and keep the same.
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- 40 percent of any amount is 40.
- A 25 percent discount takes $25 off whatever the price.
Safety card
Hazards
- water spills
Controls
- work over a tray
Note
No chemicals or heat are used, so the NSW Department of Education Chemical Safety in Schools package is not needed; ordinary classroom supervision under the school's usual risk management.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
Sources
The pages the author read to write this activity.