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Class 5 · Mathematics and money

Which pack is the better buy?

A small pack costs less money. Does that mean it is cheaper?

What you will learn: Compare prices fairly using price per 100 g.

Small test at the end: scroll down to the end of this page for five short questions that check what you have learnt. Tap an answer, then press Check answer. Go to the questions ↓

Jump to: Check yourself · For grown-ups · For teachers

Safety first

No shopping needed. Use old packets or the table.

Keep it safe:

  • No shopping is needed. Use old packets at home or the prices given here.

You will need

  • Two or three packets of the same food in different sizesOr the price list on this page.
  • Paper and a pencil

Time: About 35 minutes.

The story

At the shop, Tanmay picks up the small packet of poha. "This one is cheapest. Only ₹30!" Dadi taps a bigger packet. "That one costs more, but look how much more is inside." Tara takes out a pencil. Which packet really gives the most poha for the money?

The idea

Packs come in different sizes, so the price alone does not tell you which is cheaper.

To compare fairly, work out the price for the same amount, such as 100 g.

Price per 100 g = price ÷ (grams ÷ 100). A 500 g pack is 5 lots of 100 g, so ₹60 ÷ 5 = ₹12 per 100 g.

The lowest price per 100 g is the best value, but only if you can use it all before it goes stale. That is a trade-off.

A table of three packs. 500 grams for 60 rupees is 12 rupees per 100 grams. 200 grams for 30 rupees is 15 rupees per 100 grams. 1 kilogram for 110 rupees is 11 rupees per 100 grams.Price for the same amountPackPricePrice per 100 g500 g₹60₹12200 g₹30₹151 kg₹110₹11
A table of three packs. 500 grams for 60 rupees is 12 rupees per 100 grams. 200 grams for 30 rupees is 15 rupees per 100 grams. 1 kilogram for 110 rupees is 11 rupees per 100 grams.

Worked example

Meena compares packs of poha

  1. 500 g for ₹60: 500 g is 5 lots of 100 g, so ₹60 ÷ 5 = ₹12 per 100 g.
  2. 200 g for ₹30: 2 lots of 100 g, so ₹30 ÷ 2 = ₹15 per 100 g.
  3. 1 kg (1,000 g) for ₹110: 10 lots of 100 g, so ₹110 ÷ 10 = ₹11 per 100 g.
  4. The 1 kg pack is cheapest per 100 g, but her family uses only about 500 g a month.

She chose the 500 g pack: good value, and it gets used up before it goes stale.

Try it

  1. Collect prices.

    Write the size (in grams) and price of each pack.

  2. Find the number of 100 g lots.

    Grams ÷ 100. For 1 kg, remember 1 kg = 1,000 g.

  3. Work out price per 100 g.

    Price ÷ number of 100 g lots.

  4. Choose and give a reason.

    Which is cheapest per 100 g?

    Would your family use it all? Write your choice and reason.

What you make (paper or offline)

Your best-buy table

A table comparing three pack sizes by price per 100 grams.Price for the same amountPackPricePrice per 100 g500 g₹60₹12200 g₹30₹151 kg₹110₹11
A table comparing three pack sizes by price per 100 grams.

Each pack's size, price and price per 100 g.

Your choice, with one reason about value and one about how much you need.

Make it on paper. Keep it with you. Nothing to upload.

Check yourself

Five questions about what this page teaches. Choose an answer and press Check answer, or open "See the answer and why". Nothing is saved or sent: your choices stay on this screen and disappear when you leave or reload the page.

  1. Question 1 of 5A 250 g pack costs ₹40. What is its price per 100 g?

    See the answer and why
    • A. ₹16 Correct. 250 g is 2.5 lots of 100 g, and ₹40 ÷ 2.5 = ₹16.
    • B. ₹25 Not this one. That divides 250 by 10. Divide the price, ₹40, by 2.5.
    • C. ₹40 Not this one. ₹40 is the price of the whole 250 g pack.
    • D. ₹10 Not this one. That would be ₹40 ÷ 4, but 250 g is 2.5 lots of 100 g.
  2. Question 2 of 5Pack A costs ₹12 per 100 g. Pack B costs ₹15 per 100 g. Which is better value?

    See the answer and why
    • A. You cannot tell without the pack sizes Not this one. Price per 100 g already makes the comparison fair.
    • B. Pack A Correct. It costs less for the same amount, 100 g.
    • C. Pack B Not this one. Pack B costs ₹3 more for every 100 g.
    • D. Whichever pack is bigger Not this one. Size alone does not tell you. Price per 100 g does.
  3. Question 3 of 5Why is comparing only the prices on the packs not fair?

    See the answer and why
    • A. The packs hold different amounts Correct. A cheap small pack can cost more for each 100 g than a bigger one.
    • B. Prices change every day Not this one. Even on the same day, different sizes need a fair comparison.
    • C. The shopkeeper decides what is fair Not this one. You can check it yourself with price per 100 g.
    • D. Big packs are always cheaper Not this one. Not always. You need to check the price per 100 g.
  4. Question 4 of 51 kg of rice costs ₹90. What is the price per 100 g?

    See the answer and why
    • A. ₹900 Not this one. That multiplies instead of dividing.
    • B. ₹90 Not this one. ₹90 is the price of the whole kilogram.
    • C. ₹9 Correct. 1 kg is 1,000 g, which is 10 lots of 100 g. ₹90 ÷ 10 = ₹9.
    • D. ₹0.90 Not this one. That divides by 100. 1 kg has 10 lots of 100 g, so divide by 10.
  5. Question 5 of 5The 1 kg pack is cheapest per 100 g, but your family uses only 200 g a month and it goes stale in two months. What is the wise choice?

    See the answer and why
    • A. The 1 kg pack, because it is always the best Not this one. The lowest price per 100 g only helps if you use it all.
    • B. Whichever pack has the brightest colours Not this one. Packaging colour does not tell value.
    • C. A smaller pack you will finish before it goes stale Correct. Food that goes stale and is thrown away is wasted money.
    • D. Two 1 kg packs, to save even more Not this one. Even more of it would go stale.

Think back

Was the biggest pack always the cheapest per 100 g?

What else, besides price, matters when choosing?

Investigate further

  • Compare price per 100 mL for two sizes of oil or milk.
  • Ask a grown-up how they choose between pack sizes.
  • Refill packs: 750 mL of soap for ₹90 or 1 L for ₹110. Which is cheaper per litre (₹120 or ₹110)? If you only have ₹100 today, which can you buy? Cheapest per litre is not always the one you can afford.

For grown-upsTap a heading to open it. The safety notes are at the top of the page.

Before you start
  • Collect two or three empty packets of one food in different sizes, with prices.
Questions that help
  • "How many lots of 100 g are in this pack?"
  • "Will we use it all before it goes stale?"
What's really going on

Unit price (price per 100 g or per kg) is often printed on shop labels. Working it out by hand shows why it matters.

The best value depends on needs too. The trade-off between cost and waste is a real decision families make.

Other ways to do it
  • Use the three packs in the table on this page.

For teachersLearning aims, classroom ideas and what to look for.

Learning aims
  • Compare prices using a common unit (price per 100 g).
  • Make and justify a choice that weighs value against need.
Subject links
  • Mathematics: division, money and units of mass.
  • Social science: being a careful consumer.

These links are general. They have not been checked against a specific board syllabus.

In the classroom
  • Bring empty packets; groups make a best-buy table for one food and present their choice.
What to look for
  • Converts kg to g correctly.
  • Gives a reason about need as well as value.
ProLearners Assist