Practice Maths

L30 — Determining Best Buys

Key Terms

unit price
The cost per one unit of measurement (e.g. per gram, per litre, per item). Formula: Unit Price = Total Price ÷ Quantity.
better buy
The product with the lower unit price gives better value for money.
rate
A comparison of two quantities with different units, e.g. $3.20 per 400 g or $1.50 per litre.

Key Method

To compare products: calculate the unit price for each, then choose the lower one.

Unit Price = Total Price ÷ Quantity   (make sure both are in the same unit first!)

Worked Example

Juice A: 1.5 L for $2.40    Juice B: 2 L for $3.20    Which is better value?

Step 1 — Juice A unit price: $2.40 ÷ 1.5 = $1.60 per litre

Step 2 — Juice B unit price: $3.20 ÷ 2 = $1.60 per litre

Step 3 — Compare: Both are the same price per litre — equal value.

What Is a Unit Price?

A unit price is the cost per single unit of measurement — per gram, per millilitre, per item, etc. It lets you compare products of different sizes on a fair basis.

Formula: Unit price = Total price ÷ Quantity

  • A 500 g bag of rice for $2.50: unit price = $2.50 ÷ 500 = $0.005 per gram = 0.5 cents/g
  • A 1.2 kg bag of rice for $5.40: unit price = $5.40 ÷ 1200 = $0.0045 per gram = 0.45 cents/g

The 1.2 kg bag has a lower unit price, so it is the better buy (if you need that much).

Comparing Unit Rates Step by Step

When comparing two products:

  1. Convert to the same unit (both in grams, both in mL, etc.).
  2. Calculate the unit price for each (price ÷ quantity).
  3. The lower unit price = better value.

Example: Juice A: 600 mL for $2.40. Juice B: 1.5 L for $5.25.

  • Convert: Juice B = 1500 mL for $5.25
  • Juice A: $2.40 ÷ 600 = $0.004/mL
  • Juice B: $5.25 ÷ 1500 = $0.0035/mL
  • Juice B is cheaper per mL → Juice B is better value

Bigger Is Not Always Better Value

Many people assume the bigger pack is always cheaper per unit, but this isn’t always true. The bigger pack might be:

  • A premium brand (costs more even per gram)
  • On special with a misleading “bulk deal”
  • Not practical — if you can’t use it before it expires, it’s not a saving

Always calculate the unit price to be sure.

Expressing Unit Rates per 100 g

Unit rates can be expressed per 100 g or per 100 mL (this is how most supermarket shelf labels work):

  • $2.80 for 200 g → price per 100 g = $2.80 ÷ 2 = $1.40 per 100 g
  • $3.60 for 300 g → price per 100 g = $3.60 ÷ 3 = $1.20 per 100 g
Always convert to the same unit before comparing. Comparing $/gram with $/kg will give a misleading result. 1 kg = 1000 g, 1 L = 1000 mL.

  1. Calculate the unit price

    1. 400 g of chips for $3.20
    2. 2 L of milk for $3.00
    3. 5 kg of rice for $8.50
    4. 12 eggs for $4.80
    5. 3 kg of potatoes for $4.20

  2. Which is the better buy?

    Calculate the unit price for each option and state which gives better value.

    1. 200 g for $1.60  or  300 g for $2.10
    2. 1.5 L for $2.25  or  2 L for $3.20
    3. 750 g for $4.50  or  1 kg for $5.80

  3. Chocolate Bars

    A 250 g block of chocolate costs $4.50. A 400 g block costs $6.80. Which is better value per gram?

  4. Shampoo Comparison

    Shampoo A: 500 mL for $7.50. Shampoo B: 750 mL for $10.50. Which has the lower unit price?

  5. Orange Juice

    Brand X: 2 L for $3.80. Brand Y: 3 L for $5.40. Which is the better buy?

  6. Eggs: Singles vs Dozen

    Eggs are sold individually at 60c each, or in a dozen for $6.60. Which option gives a better price per egg?

  7. Budget Shopping

    You need 1 kg of flour. Option A: 500 g for $1.40. Option B: 1 kg for $2.50. Which should you buy? How much do you save?

  8. Coffee Pods

    Pack A: 10 pods for $8.00. Pack B: 16 pods for $12.00. Pack C: 24 pods for $17.40. Rank from cheapest to most expensive per pod.

  9. Price Per 100 g

    Cereal Brand X: 375 g for $5.25. Cereal Brand Y: 500 g for $6.50. Calculate the price per 100 g for each. Which is cheaper?

  10. Fair Price

    If 600 g of pasta costs $2.40, what would be a fair price for a 1 kg packet at the same unit price?

See Answers ➔