What it shows
The cheapest looking pack is not always the best buy. Dividing the price by the weight or volume gives the unit price, which lets you compare packs of different sizes fairly.
The science
The unit price is the price divided by the quantity, such as rupees per 100 g or per litre. It converts packs of different sizes to a single common measure, so you can compare them. Larger packs are often cheaper per unit, but not always, and a cheap price is no gain if the food spoils before it is used.
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 or sheet with a table
- A calculator
- Prices and pack sizes of five items such as rice, dal, oil, biscuits and soap
- Small and big pack of each, noted from a shop or from home
- A pencil
- Graph paper
- A kitchen scale for checking weights
Safety
For this project: Go to the shop with an adult and do not touch things without permission. Note prices only and do not collect personal information.
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.
Choose five items and note the price and weight of two or three pack sizes of each.
Convert all weights to the same unit, either grams or kilograms.
Divide the price by the weight to get the price per 100 g, or per litre for oil.
Write the unit prices in a table beside the pack size.
Mark the best buy in each row and calculate the saving if the family always buys it.
Find any pack that is not the best buy even though it is larger.
Draw a bar chart of unit prices for one item and write a short advice note for your family.
What you should see
Your table shows the unit price of every pack and identifies the best buy for each item. In most items a larger pack is cheaper per unit, but at least one exception may show the opposite.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is a unit price?
The cost of one standard unit of weight or volume.
2Why compare unit prices?
To compare packs of different sizes fairly.
3Is the larger pack always cheaper per unit?
No, check each time.
4What is the price per 100 g of a 500 g pack costing Rs 60?
Rs 12.
Take it further
Compute the yearly saving for your home if it buys the best buy of each of ten items.
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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