Best buys and value for money
Maths · Key Stage 3 · The School
Why total price misleads
A 200g bag of rice costs £1.20. A 500g bag costs £2.75. The 500g bag costs more in total, so it looks worse — but it is a much bigger bag. Comparing total prices only works when the amounts are the same, and here they are not.
Finding the unit price
Divide price by amount to get the UNIT PRICE — the cost of a single gram. The 200g bag: £1.20 ÷ 200 = £0.006 per gram. The 500g bag: £2.75 ÷ 500 = £0.0055 per gram. The 500g bag is actually cheaper per gram, even though its total price is higher.
Comparing offers, not just packs
The same method compares "3 for £5" against "buy one get one half price", or a multipack against single items. Work out what one unit truly costs under each offer, then compare those single numbers directly — never the headline price or the words on the label.
Bottle A: 250ml for £2.00. Bottle B: 400ml for £3.00. Bottle C: 600ml for £4.20. Unit price A: 2.00 ÷ 250 = £0.008/ml. Unit price B: 3.00 ÷ 400 = £0.0075/ml. Unit price C: 4.20 ÷ 600 = £0.007/ml. Bottle C has the highest total price of the three, and is still the best value, at £0.007 per ml. The bottle that costs the most is the one that costs the least per millilitre — which is exactly why the unit price, not the total price, is the number that matters.