Best buys

The Maths Habit

  1. 5 mugs cost £8.50. How much do 3 mugs cost?

    £5.10

  2. A recipe for 8 people uses 600 ml of stock. How much stock is needed for 12 people?

    900 ml

  3. A plumber charges £40 for 1 hour and £70 for 2 hours. Is the charge directly proportional to the time?

    No: 2 hours would be £80

Box A: 500 g of cereal for £2.
Box B: 1 kg of cereal for £4.20.
Which would you buy? Why?

Two of box A is 1 kg for £4, so A is better value.

Some people still choose B: one box, less packaging.

Better buy
The one that gives more for the same money.
Unit cost
The cost of one unit.

Bag A: 600 g of pasta for £1.32. Bag B: 1 kg of pasta for £2.10. Which is the better buy?

PriceAmountCost of 100 gBetter buy
A£1.32600 g£1.32 ÷ 6 = 22p
B£2.101 kg£2.10 ÷ 10 = 21p✓

1 Make the amounts the same. 2 Compare the prices. The lower cost is the better buy.

Box A: 2 kg of dog biscuits for £5.40. Box B: 5 kg for £14. Find the cost of 1 kg for each.

PriceAmountCost of 1 kgBetter buy
A£5.402 kg£5.40 ÷ 2 = £2.70✓
B£145 kg£14 ÷ 5 = £2.80

Bag A: 600 g of pasta for £1.32. Bag B: 1 kg of pasta for £2.10. Which is the better buy?

PriceAmountCost of 100 gBetter buy
A£1.32600 g£1.32 ÷ 6 = 22p
B£2.101 kg£2.10 ÷ 10 = 21p✓

Pack A: 12 bin bags for £1.80. Pack B: 20 bin bags for £2.80. Find the cost of 1 bag for each.

PriceAmountCost of 1 bagBetter buy
A£1.801215p for 1 bag
B£2.802014p for 1 bag✓

Box A: 1.2 kg of washing powder for £3. Box B: 2 kg for £4.80.

PriceAmountAmount for £1Better buy
A£31.2 kg1200 ÷ 3 = 400 g
B£4.802 kg2000 ÷ 4.80 = 416.7 g✓

Find the amount you get for £1. The higher amount is the better buy.

Pack A: 750 g for £3. Pack B: 400 g for £1.80. Find the cost of 100 g. Then find the amount for £1.

PriceAmountCost of 100 gAmount for £1
A£3750 g40p250 g
B£1.80400 g45pabout 222 g

Both say A.

Box A: 1.2 kg of washing powder for £3. Box B: 2 kg for £4.80.

PriceAmountAmount for £1Better buy
A£31.2 kg1200 ÷ 3 = 400 g
B£4.802 kg2000 ÷ 4.80 = 416.7 g✓

Bottle A: 500 ml for £1.25. Bottle B: 750 ml for £2. Find the amount you get for £1.

PriceAmountAmount for £1Better buy
A£1.25500 ml400 ml✓
B£2750 ml375 ml

Box A: 1.2 kg of washing powder for £3. Box B: 2 kg for £4.80.

PriceAmountAmount for £1Better buy
A£31.2 kg1200 ÷ 3 = 400 g
B£4.802 kg2000 ÷ 4.80 = 416.7 g✓

Tub A: 300 g of oats for £1.50. Tub B: 500 g for £2. Tub C: 900 g for £4. Find the amount you get for £1. Which is the best buy?

PriceAmountAmount for £1Better buy
A£1.50300 g200 g
B£2500 g250 g✓
C£4900 g225 g

Pack A is 300 g for £2.10. Pack B is 500 g for £3.20. Which is better value?

AB. 100 g of B costs 64p. 100 g of A costs 70p.
BA. £2.10 is less than £3.20.
CA. £1 buys 143 g of A and 156 g of B. The lower number is the better buy.
DB. It is 200 g more for only £1.10 more.
A: rightB: compared the prices only: the amounts are not the sameC: used the cost rule on the amount for £1: for the amount for £1, the higher amount is the better buyD: compared the differences: make the amounts the same first

Best buys worksheet, page 1

Answer sheet

  1. Jar A: 360 g of jam for £2.30. Jar B: 450 g for £2.86. Which is the better buy?

    A 63.89p, B 63.56p for 100 g: B

  2. The exchange rate is £1 = 1.80 Canadian dollars. A jacket costs 63 Canadian dollars in Canada and £36 in the UK. Where is it cheaper?

    63 ÷ 1.80 = £35: Canada, by £1

  1. Tin A: 400 g for £1.20. Tin B: 250 g for 80p. Find the cost of 100 g of each. Which is the better buy?

    A: 30p and 32p

  2. Bag A: 2 kg for £3.50. Bag B: 1.5 kg for £2.40. Find the amount you get for £1. Which is the better buy?

    B: about 571 g and 625 g

  3. Pack A: 8 rolls for £3.60. Pack B: 4 rolls for £1.90. Lily says B is better value because it costs less. Is she right? Explain.

    No: A is 45p a roll, B is 47.5p
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