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. This topic appears less often, but it can still be a useful differentiator on mixed-topic papers. It is section 7 of 7 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 7
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