A price tells you what you hand over. It does not tell you what you are getting — and the number that does is printed small in the corner of almost every shelf label in the country. Twenty-four cards, a quiz in three tiers and four workshop activities cover price per unit, needs against wants, saving towards one thing, and the moment the cheaper-per-unit rule stops being the right answer.
Two boxes of the same cereal. The small one is $3. The big one is $5. Most people look at those two numbers and say the small one is cheaper.
A price tells you what you hand over. It does not tell you what you are getting, and those are different questions. The number that answers the second one is the price per unit — per gram, per liter, per sheet — and it is printed small on almost every shelf label in the country because a law says it has to be. Learning to find it takes about a minute and it is the single most useful piece of arithmetic anybody will teach you this year. But it is a tool for COMPARING, not a rule about what to buy, and the next part of this room is about when it stops being the right answer.
Two things people get wrong
⚠️People often think…
The cheaper price is the better deal.
A price is only half an answer, because it tells you what leaves your hand and says nothing about what arrives in it. Two things have to be compared: what it costs, and how much of it there is. A $5 box with 750 grams in it beats a $3 box with 300 grams, and it is not close — you are paying about a third less for every gram. The math is a single division, and on nearly every shelf somebody has already done it and printed it in the corner of the label. Once you can find that small number, whole aisles stop being confusing.
Price is half an answer. Price per unit is the whole one.
⚠️People often think…
So anybody buying the small box is just being careless with money.
This is the most important thing in this room and it is the part usually left out. The big box is only the better deal if you have the bigger price today. If there is $4 in the house and the big box costs $5, then the big box is not a choice at all — and the family who buys the small one is not being careless, they are doing arithmetic with a number you cannot see. The same goes for a fridge that is too small to store it, a walk home with only two hands, or a food that would go off before it was finished. The cheaper-per-unit rule assumes you can pay more now to spend less later, and not everybody can. So the rule is real, and it is not a way of judging anybody. Somebody buying small is usually the person doing the hardest arithmetic in the shop.
Cheaper per unit assumes you can pay more today. Not everybody can.
Compare two things properly
1Watch one
Six rolls of kitchen towel for $6, or ten rolls for $9. Which is cheaper per roll?
Do not compare $6 and $9. Those are different amounts of towel.
Divide each price by how many rolls: 6 ÷ 6, and 9 ÷ 10.
That gives $1.00 a roll, and $0.90 a roll.
The ten-pack is cheaper per roll, even though it costs more at the till.
Ten rolls for $9. ✓ And the second question is always: do I have the $9 today, and where would I put it?
2Do one with me
Fill in the three boxes.
4 cans for $8. What is the price per can?
The same brand: 6 cans for $9. Price per can?
Name ONE reason somebody might still buy the 4-pack.
💬One sentence, then you move on
Why is a price on its own only half an answer?
3Try one
A family buys the small box even though the big one is cheaper per gram. Give one good reason — not a careless one.
I want a hint first
The rule says spend more now to spend less later. What has to be true for “spend more now” to be possible at all?
💬Last one — then you’re done here
Think of something you wanted a lot and then did not want a month later. What do you think changed?
Where this goes
Where this lives
Every shelf in every shop, for the rest of your life. Also phone plans, tickets, refills and anything sold in more than one size — which is nearly everything.
What this feeds
Next room: planning, cooking and clearing up one whole meal — where the shopping list you cost out is the first step. Then in grade 11, Independent Living and Money does budgets, wages and rent properly.
Next time you are in a shop, find the small price-per-unit number on a shelf label. Write down what it said and what it was per.
One card at a time — tap “Show me” to check yourself, then Next. Start at Foundation; when those feel easy, climb.
Helpful Hints
🧭 The unit in one line
A price tells you what you hand over, not what you get → divide the price by the amount and you have the price per unit, which is already printed small on almost every shelf label → bigger is usually but not always cheaper per unit → and the rule quietly assumes you can pay the bigger price today, have somewhere to keep it, and will use it before it goes off.
Divide price by amount. Then ask what the rule assumed.
🔑 The one idea
There is a piece of arithmetic here that takes one minute to learn and is worth money every week for the rest of your life, and there is a second thing that matters more. The arithmetic is one division. The second thing is that the answer it gives you is about the PRODUCT, not about the person holding it. Nearly every version of this lesson teaches the division and stops, and a child who learns only that half walks out able to look down on their own family’s shopping. So this room teaches both halves, and the second one is the one to keep.
⚠️ Traps to avoid
A price on its own is half an answer. You need the price AND how much is in it.
The bigger pack is USUALLY cheaper per unit — not always. Shops price one the other way often enough to be worth checking.
Cheaper per unit assumes you can pay the bigger price TODAY. Somebody buying small is usually doing the harder arithmetic.
A bargain you throw away because it went off is the most expensive option there was.
Half price on something you were not going to buy is not saving half — it is spending all of it.
Eye level on a shelf holds the most expensive versions, not the cheapest. Look up and down.
You can only compare unit prices that are in the SAME units. Cents per gram against dollars per kilo is not a comparison yet.
📐 The sum, and the question after it
The choice
The sum
What it tells you
The question after
['$3 / 300g vs $5 / 750g', '$3 / 300g vs $5 / 750g']
['price ÷ grams', 'precio ÷ gramos']
['1.0¢/g vs 0.7¢/g', '1.0¢/g vs 0.7¢/g']
do I have the $5 today?
['4 cans $8 vs 6 cans $9', '4 latas $8 vs 6 latas $9']
['price ÷ cans', 'precio ÷ latas']
['$2 vs $1.50 a can', '$2 vs $1.50 por lata']
where would I put six?
['6 rolls $6 vs 10 rolls $9', '6 rollos $6 vs 10 rollos $9']
['price ÷ rolls', 'precio ÷ rollos']
['$1.00 vs $0.90 a roll', '$1.00 vs $0.90 por rollo']
can I carry it home?
['cheap shoes vs good shoes', 'zapatos baratos vs buenos']
['the real unit price', 'el precio por unidad real']
will it be finished in time?
['half price offer', 'oferta a mitad de precio']
['—', '—']
['nothing, on its own', 'nada, por sí sola']
was it on my list?
🎯 How you will be asked
“Which is cheaper per unit?” — with two sizes and two prices given.
“Find the unit price on this shelf label.”
“Need or want?” — and the ones that are genuinely arguable.
“Give a good reason somebody buys the smaller size.”
“What does the cheaper-per-unit rule assume?”
✅ Before the test, can you…
Work out a price per unit from a price and an amount?
Find the printed unit price on a real shelf label?
Say the difference between a need and a want, and name one that could be either?
Name two things the cheaper-per-unit rule assumes?
Explain why it is a tool for comparing and not for judging?
📚 Go to the sources
These open another website and need the internet. The first one is the actual rule that makes shops print the unit price — it is worth knowing it is a law and not a favor. The third is for a grown-up to look at with you.
Look back at anything you missed — the hint that appeared is exactly what to reread tonight.
How sure did you feel?
Workshop
Sort fourteen things into needs and wants and find the two that are genuinely arguable, read a real shelf label, put the six steps of comparing two sizes back in order, and then agree with somebody’s arithmetic while refusing their conclusion.
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