What does "best buy" mean?
A "best buy" question asks which of two or more products offers the best value for money — not which one is cheapest overall. A bigger pack is not automatically better value; it depends on the price per unit.
The key idea: convert every option to the same unit (e.g. price per 100g, per litre, or per item) before comparing them.
The unit price method
Worked Example 1
A 500g bag of rice costs £2.00. A 750g bag of the same rice costs £2.25. Which bag is better value?
1
Find the price per 100g of the 500g bag: £2.00 ÷ 500 × 100 = £0.40
2
Find the price per 100g of the 750g bag: £2.25 ÷ 750 × 100 = £0.30
3
Compare the two unit prices: £0.30 is less than £0.40, so the 750g bag is cheaper per 100g
AnswerThe 750g bag is better value
Dividing each bag into actual 100g chunks (5 chunks vs 7.5 chunks) shows the price of one chunk directly, rather than relying on an abstract bar length.
Comparing different pack sizes
The same method works for any unit — per item, per litre, per kilogram — as long as both options are converted to the same unit before comparing.
Worked Example 2
A 6-pack of yoghurts costs £2.40. A 9-pack of the same yoghurts costs £3.15. Which pack is better value?
1
Find the price per yoghurt in the 6-pack: £2.40 ÷ 6 = £0.40
2
Find the price per yoghurt in the 9-pack: £3.15 ÷ 9 = £0.35
3
Compare: £0.35 is less than £0.40, so the 9-pack is cheaper per yoghurt
AnswerThe 9-pack is better value
The amount-per-£1 method
An alternative approach is to find how much product you get for £1 — the option that gives you more for your pound is the better value. This is especially useful when prices are round numbers.
Worked Example 3
Two bags of the same coffee are on sale: 250g for £4.25, and 400g for £6.80. Which is better value?
1
Find the price per 100g of the 250g bag: £4.25 ÷ 250 × 100 = £1.70
2
Find the price per 100g of the 400g bag: £6.80 ÷ 400 × 100 = £1.70
3
Compare: both bags cost exactly £1.70 per 100g
AnswerBoth bags offer equal value
Not every best buy question has a clear winner — sometimes, as above, two options genuinely offer identical value. Always check your working rather than assuming the bigger pack must win.
Practice questions
Work through each question before checking the answers.
Foundation (Grade 3–5)
Q1A 400g jar of jam costs £2.00. A 600g jar costs £2.70. Find the cost per 100g of each, and state which is better value.Foundation
Q2A pack of 4 pens costs £1.20. A pack of 10 pens costs £2.50. Which pack is better value per pen?Foundation
Q32 litres of juice costs £3.00. 3 litres costs £4.20. Which is better value per litre?Foundation
Q4A small box of cereal (250g) costs £1.75. A large box (500g) costs £3.20. Which is better value?Foundation
Q55 apples cost £1.00. 8 apples cost £1.44. Which offers better value per apple?Foundation
Higher (Grade 5–7)
Q6A 750ml bottle of shampoo costs £4.50. A 1 litre bottle costs £5.60. Find the cost per 100ml of each, and state which is better value.Higher
Q7A 12-pack of eggs costs £2.88. A 6-pack costs £1.62. Which pack is better value per egg?Higher
Q81.5kg of flour costs £2.10. 2.5kg costs £3.00. Which is better value per kg?Higher
Q9A café sells a small coffee (200ml) for £2.20, and a large coffee (350ml) for £3.50. Which size gives better value per ml?Higher
Q10A box of 40 tea bags costs £2.80. A box of 100 tea bags costs £6.50. Which box offers better value per tea bag?Higher
Higher — Hard (Grade 8–9)
Q11Brand A sells 300ml of sauce for £1.80. Brand B sells 450ml of the same sauce for £2.52. Which brand offers better value, and by how much per 100ml?Grade 8–9
Q12A supermarket sells a 6-pack of soda for £3.30, and a 24-pack for £12.48. Which pack offers better value per can, and what is the price difference per can?Grade 8–9
Q13Two bags of the same coffee: 250g for £4.25, and 400g for £6.80. Determine which is better value.Grade 8–9
Q14A shop sells notebooks in packs: a pack of 3 costs £2.40, and a pack of 5 costs £3.75. A customer needs exactly 15 notebooks. Compare buying five 3-packs against buying three 5-packs, and state which is cheaper.Grade 8–9
Q15A 2kg bag of potatoes costs £1.80. A 5kg bag costs £4.00. Loose potatoes at a market stall cost 85p per kg. Rank all three options from best to worst value per kg.Grade 8–9
Answers
Foundation (Q1–Q5)
Q1400g: £0.50/100g, 600g: £0.45/100g — 600g jar is better value
Q24-pack: £0.30/pen, 10-pack: £0.25/pen — 10-pack is better value
Q32L: £1.50/L, 3L: £1.40/L — 3L is better value
Q4250g: £0.70/100g, 500g: £0.64/100g — 500g box is better value
Q55-pack: £0.20/apple, 8-pack: £0.18/apple — 8-pack is better value
Higher (Q6–Q10)
Q6750ml: £0.60/100ml, 1L: £0.56/100ml — 1L bottle is better value
Q712-pack: £0.24/egg, 6-pack: £0.27/egg — 12-pack is better value
Q81.5kg: £1.40/kg, 2.5kg: £1.20/kg — 2.5kg is better value
Q9Small: 1.1p/ml, Large: 1.0p/ml — large coffee is better value
Q1040-box: 7p/bag, 100-box: 6.5p/bag — 100-box is better value
Higher — Hard (Q11–Q15)
Q11Brand B is better value, by £0.04 per 100ml(A: £0.60/100ml, B: £0.56/100ml)
Q1224-pack is better value, by £0.03 per can(6-pack: £0.55/can, 24-pack: £0.52/can)
Q13Equal value — both £1.70 per 100g
Q14Three 5-packs is cheaper, saving £0.75(five 3-packs = £12.00, three 5-packs = £11.25)
Q15Best to worst: 5kg bag (£0.80/kg), loose (£0.85/kg), 2kg bag (£0.90/kg)
Common mistakes
Common Mistake 1
Comparing total prices instead of unit prices
A £4.20 bottle is not automatically worse value than a £3.00 bottle — it depends how much product each one contains. Always compare price per unit, never the total price alone.
Common Mistake 2
Comparing different units without converting first
You can't directly compare "price per kg" with "price per 100g" — convert both options to the exact same unit before making any comparison.
Common Mistake 3
Rounding too early
Rounding a unit price too soon can hide a genuine difference between two close options — keep at least 2–3 decimal places (or work in pence) until the final comparison.
Common Mistake 4
Assuming the bigger pack is always better value
Bulk packs are usually — but not always — better value. Special offers, promotions, or non-linear pricing can mean a smaller pack is occasionally the cheaper option per unit.
Exam tips
💡 Exam Tip 1
Always state your chosen unit clearly
Write "price per 100g" or "price per item" as a clear label on your working — this shows the examiner exactly what you're comparing and picks up method marks.
💡 Exam Tip 2
Work in pence when prices are close
Converting pounds to pence (e.g. £0.056 → 5.6p) makes small differences between unit prices much easier to compare accurately.
💡 Exam Tip 3
Always state the final conclusion in words
"£0.30 per 100g" is not a complete answer on its own — finish with a sentence stating which option is better value, to secure the final accuracy mark.
💡 Exam Tip 4
Check whether the question needs a total, not a unit price
Some questions (like buying multiple packs to reach an exact quantity) need a comparison of total cost for a fixed amount — read carefully to see which comparison is actually being asked for.
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