What it shows
Two successive discounts do not add up to one big discount. A 20 percent off followed by 10 percent off is worth 28 percent, not 30. You test this with real prices from a sale and learn to calculate discounts correctly.
The science
A 20 percent discount leaves 80 percent of the price. A second discount of 10 percent is then taken from the new, lower price and leaves 90 percent of that, so the final price is 0.8 times 0.9, which is 72 percent of the original. The total saving is 28 percent, as the second percentage applies to a smaller base.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A notebook and a pencil
- A calculator
- Price tags or newspaper advertisements of five items
- A ruler
- Graph paper
- Play money or coins
- A friend to check your work
Safety
For this project: No special safety needed. Work with real prices only from advertisements, and never share personal details.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
7 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Pick an item priced at Rs 1,000 and calculate its price after a 20 percent discount.
Take 10 percent off that new price and write the final price.
Calculate the price after a single discount of 30 percent and compare it.
Do the same for three other price tags from advertisements.
Swap the order, taking 10 percent first and then 20 percent, and see whether the result changes.
Write down the multiplier for each case and work out the combined discount.
Find the single discount equal to two discounts of 15 percent each and explain it.
What you should see
Two discounts of 20 and 10 percent give a total of 28 percent, and the order of the two does not change the final price. A single 30 percent discount is better for the buyer.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Is 20 percent then 10 percent the same as 30 percent?
No, it equals 28 percent.
2Why not?
The second discount applies to a smaller price.
3Does the order matter?
No, the multipliers 0.8 and 0.9 can be multiplied in either order.
4What is the price after 15 percent off Rs 400?
Rs 340.
Take it further
Find out what a Rs 500 item costs with GST added before and after a discount, and say which is cheaper.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
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