This study notes covers Common Mistakes within Best Buy for GCSE Mathematics. Revise Best Buy in Ratio & Proportion for GCSE Mathematics with 12 exam-style questions and 22 flashcards. Use this page as part of a wider topic revision path rather than treating it as an isolated fact. It is section 7 of 8 in this topic. Use this study notes to connect the idea to the wider topic before moving on to questions and flashcards.
Topic position
Section 7 of 8
Practice
12 questions
Recall
22 flashcards
Common Mistakes
✗ Comparing total prices without considering quantities
✓ Always calculate price per unit (per kg, per litre, etc.)
✗ Forgetting to account for "free" items in multi-buy offers
✓ In "3 for 2", you get 3 items but pay for 2
✗ Not converting to same units (comparing pence per gram vs pounds per kg)
✓ Use consistent units throughout your calculation
Practice Questions
Q1 Rice: Small pack 500g for £1.80, Large pack 1.2kg for £4.50. Which is better value? [2 marks]
[1] Small: £1.80 ÷ 500g = £0.0036/g, Large: £4.50 ÷ 1200g = £0.00375/g
[1] Small pack better value (£0.0036 < £0.00375)
Q2 Orange juice: 1 litre for £2.40 or 1.5 litres for £3.30. Which is better value? [2 marks]
[1] 1L: £2.40/L, 1.5L: £3.30 ÷ 1.5 = £2.20/L
[1] 1.5L better value
Q3 Bread: 400g for £1.20 or 800g for £2.00. Which is better value? [2 marks]
[1] 400g: £1.20 ÷ 400 = £0.003/g, 800g: £2.00 ÷ 800 = £0.0025/g
[1] 800g better value
Q4 Chocolate bars: £1.50 each OR 3 for £4.00. Which is better value? [2 marks]
[1] Individual: £1.50 each, Multi-buy: £4.00 ÷ 3 = £1.33 each
[1] 3 for £4.00 better value
Q5 Trainers: Normal price £80. Shop A: 25% off. Shop B: £65. Which is better? [3 marks]
[1] Shop A discount: 25% of £80 = £20
[1] Shop A price: £80 - £20 = £60
[1] Shop A better (£60 < £65)
Q6 Yogurts: Buy 4 get 1 free at £0.80 each OR Buy any 3 for £2.00. Which is better? [3 marks]
[1] Offer 1: Pay 4 × £0.80 = £3.20, get 5 yogurts = £0.64 each
[1] Offer 2: £2.00 ÷ 3 = £0.67 each
[1] Buy 4 get 1 free better value
Q7 Washing powder: 2kg for £8.50 or 500g for £2.40. Which is better value? [3 marks]
[1] 2kg: £8.50 ÷ 2000g = £0.00425/g
[1] 500g: £2.40 ÷ 500g = £0.0048/g
[1] 2kg better value
Q8 Mobile phone: Store A: £450 with 15% off. Store B: £380 with £30 cashback. Which is cheaper? [3 marks]
[1] Store A: £450 × 0.85 = £382.50
[1] Store B: £380 - £30 = £350
[1] Store B cheaper
Q9 Coffee: Regular price £6.50. Offer A: Buy 2 get 1 free. Offer B: 30% off when you buy 3. Compare for buying 3 jars. [4 marks]
[1] Offer A: Pay for 2, get 3 = 2 × £6.50 = £13.00
[1] Offer B: 3 × £6.50 = £19.50
[1] 30% off: £19.50 × 0.7 = £13.65
[1] Offer A better (£13.00 < £13.65)
Q10 Paint: 2.5L for £18.75 or 750ml for £6.30. Calculate the price per 100ml for each. [4 marks]
[1] 2.5L = 2500ml: £18.75 ÷ 2500ml = £0.0075/ml
[1] Price per 100ml: £0.0075 × 100 = £0.75
[1] 750ml: £6.30 ÷ 750ml = £0.0084/ml
[1] Price per 100ml: £0.0084 × 100 = £0.84
Q11 Pizza: Large (12 inch diameter) £15.99 or Medium (9 inch diameter) £10.99. Which gives more pizza per pound? (Area = πr²) [4 marks]
[1] Large area: π × 6² = 36π square inches
[1] Medium area: π × 4.5² = 20.25π square inches
[1] Large: 36π ÷ £15.99 = 7.1 sq in/£
[1] Medium: 20.25π ÷ £10.99 = 5.8 sq in/£. Large better value
Q12 Gym membership: Annual £480, Monthly £45, or Pay-as-you-go £8 per visit. Tom plans 65 visits per year. What's cheapest? [5 marks]
[1] Annual: £480
[1] Monthly: £45 × 12 = £540
[1] Pay-as-you-go: £8 × 65 = £520
[2] Annual cheapest (£480 < £520 < £540)
Q13 Laptop: RRP £800. Store A: 20% off plus £50 cashback. Store B: 25% off. Store C: £600 with 5% extra off if you're a student. Which is cheapest for a student? [5 marks]
[1] Store A: £800 × 0.8 - £50 = £640 - £50 = £590
[1] Store B: £800 × 0.75 = £600
[1] Store C: £600 × 0.95 = £570
[2] Store C cheapest for students
Q14 Car insurance: Company A: £1200 with 15% no-claims discount and 10% online discount (discounts applied separately). Company B: £1000 flat rate. Which is cheaper? [5 marks]
[1] Company A after no-claims: £1200 × 0.85 = £1020
[1] After online discount: £1020 × 0.9 = £918
[1] Company B: £1000
[2] Company A cheaper (£918 < £1000)
Q15 Energy tariff: Standard rate 28p per kWh. Economy 7: Day rate 32p per kWh, night rate 16p per kWh. Sarah uses 200 kWh day, 100 kWh night monthly. Which tariff saves money? [6 marks] Challenge
[2] Standard: (200 + 100) × 28p = 300 × 28p = £84
[2] Economy 7: (200 × 32p) + (100 × 16p) = £64 + £16 = £80
[2] Economy 7 saves £4/month
Q16 Petrol: Station A: £1.35/L with 5p off per litre if you spend over £30. Station B: £1.32/L with loyalty card giving 2% cashback. For a 35L tank, which is cheaper? [6 marks] Challenge
[1] Station A without discount: 35 × £1.35 = £47.25 (> £30)
[1] Station A with discount: 35 × £1.30 = £45.50
[1] Station B: 35 × £1.32 = £46.20
[1] Cashback: £46.20 × 0.02 = £0.92
[1] Station B final: £46.20 - £0.92 = £45.28
[1] Station B cheaper (£45.28 < £45.50)
Q17 Phone contract: Plan A: £25/month + 10p per minute. Plan B: £40/month unlimited calls. How many minutes make Plan B better value? [7 marks] Challenge
[2] Let x = minutes used. Plan A cost: £25 + 0.10x
[2] Plan B cost: £40. For Plan B better: £40 < £25 + 0.10x
[2] £15 < 0.10x, so x > 150
[1] Plan B better if >150 minutes/month
Q18 Cinema: Tickets £12 each. Annual membership £60 gives 30% off all tickets. Premium membership £120 gives 50% off plus free popcorn (worth £4). How many visits make Premium worthwhile compared to: (a) No membership (b) Annual membership? [8 marks] Challenge
[2] Premium vs No membership: £120 + 6x + 4x vs 12x, so 10x = 120, x = 12
[2] Premium cost per visit: £6 + £4 popcorn = £10 per visit
[2] Annual cost per visit: £8.40 per visit (30% off £12)
[2] Premium vs Annual: £120 + 10x vs £60 + 8.4x, so 1.6x = 60, x = 37.5
Extra working shows full method for both comparisons
Keep building this topic
Read this section alongside the surrounding pages in Best Buy. That gives you the full topic sequence instead of a single isolated revision point.
Practice Questions for Best Buy
Which method correctly compares the value of two products?
Three sizes of shampoo are available: - Travel: 100 ml for 95p - Regular: 300 ml for £2.55 - Family: 500 ml for £4.00 Find the best value size. Show all working.
Quick Recall Flashcards
What does BOGOF mean?
Buy One Get One Free. You pay for 1 item but get 2 items. Price per item = Normal price ÷ 2.
What does 'best buy' mean?
The option that gives the lowest cost per unit (per gram, per litre, per item). Calculate by dividing total price by quantity.
12 questions on Best Buy — practise free
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