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This page sets out the method for best buy questions in the order you would use it, then walks through the full lesson explanation. For three complete worked examples with the slips that cost marks, see the Best buy questions topic page.
When the quantities are different sizes, divide each price by its quantity.
When one quantity is a simple multiple of the other, scale up to matching amounts.
Divide price by quantity.
£6 for 3 tins is £2 per tin.
Compare price per gram, or per tin, or per litre.
Do not compare a price per tin with a price per gram.
The lower unit price is the better value.
Say which offer, and show both unit prices.
You can also price the same number of units from each offer.
Whichever costs less for that common amount is the better buy.
“3 for the price of 2” changes the quantity you pay for.
Work out the real cost of one item after the offer.
Write both divisions.
A choice with no working does not earn the method marks.
Unit price puts both offers into the same comparison. A bag of 5 apples at £2 costs 2 ÷ 5 = 0.40 pounds per apple. A bag of 8 at £2.80 costs 2.80 ÷ 8 = 0.35 pounds per apple. The second bag is better value even though its price tag is higher.
The division is price divided by quantity, not the other way round, when you want a price. Keep the units the same on both offers. Price per apple against price per apple is fair. Price per apple against price per gram is not. Write both divisions before you choose.
Method: 1. Write the price and the quantity for each offer. 2. Divide price by quantity for each offer. 3. Check that both answers use the same unit. 4. Choose the lower price per unit.
Amount per pound answers a different question: how much stuff does £1 buy? Divide quantity by price. Two hundred grams for £2 is 100 g per pound. Two hundred and fifty grams for £5 is 50 g per pound. The first offer gives more cereal for each pound.
The decision flips compared with unit price. A bigger number is now better, because you are counting goods rather than cost. If you forget which way you divided, you can pick the worse offer with confident working. Label the line “grams per pound” or “pounds per gram” so the choice matches the label.
Method: 1. Divide quantity by price for each offer. 2. Label the result as an amount per pound. 3. Choose the larger amount per pound. 4. Do not compare an amount per pound with a price per unit.
Write the two unit prices one under the other, then subtract. The difference tells you how much you save on each unit, and the smaller price is the one you would pay. For the apples, 0.40 - 0.35 = 0.05, so the better bag saves five pence on every apple.
The question in this lesson asks for that cheaper unit price, or for the cost of a shared quantity, not for the name of a shop. The number is the thing a marker can check. Mention which offer it came from in the working, and box the price.
Method: 1. Calculate both unit prices. 2. Subtract to see the gap. 3. State the lower price clearly. 4. Give the answer as a number with a money unit.
Packs of 3 and packs of 5 can both make 15 cartons, because 15 is a multiple of each. Five packs of 3 cost 5 × 4.50 pounds. Three packs of 5 cost 3 × 7 pounds. The cheaper bill for the same 15 cartons is the better offer.
This route avoids a unit price altogether. It is useful when the pack sizes are awkward and a shared quantity is easy. You must compare the same quantity. A cost for 15 against a cost for 10 does not decide the better buy, because you would be buying different amounts.
Method: 1. Find a common multiple of the two pack sizes. 2. Work out how many packs of each offer make that quantity. 3. Multiply each pack price by the number of packs. 4. Choose the lower cost for that shared quantity.
A best buy question gives two or more deals and asks which is better value. You can only compare them fairly if you turn each deal into the same unit. The usual choices are the price of one item, the price of 100 g or 100 ml, or the amount you get for £1. Pick whichever gives the friendlier division, then use the same unit for every deal.
The units in the question may not match, such as one deal in kilograms and the other in grams. Convert first. A comparison across different units is the easiest way to lose the marks. Write the unit next to every answer, for example 18.5p per 100 ml, so that the comparison line is easy to read and the examiner can see that your units agree.
Method: 1. Convert any quantities to the same unit, such as grams or millilitres. 2. Choose a comparison unit, such as price per 100 g or amount per £1. 3. Work out the same measure for every deal, keeping enough digits. 4. Compare, and write a sentence naming the better deal.
Tip: Price per unit means the lower number wins. Amount per pound means the higher number wins. Never mix the two.
Shops advertise offers such as "20% extra free" or "15% off". They are not the same kind of change. "Extra free" increases the quantity you get while the price stays the same, so you multiply the quantity by 1.2. "Off" lowers the price while the quantity stays the same, so you multiply the price by 0.85. Work out the new quantity or the new price, then compare price per unit.
A shortcut is to compare multipliers on the price per unit. A "40% extra" deal divides the unit price by 1.4, which is about 0.714 of the original. A "30% off" deal multiplies by 0.7. The smaller multiplier is the better deal. Be careful that "25% extra" is not the same as "25% off": it is equal to about 20% off, because 1 ÷ 1.25 = 0.8.
Method: 1. For "extra free", multiply the quantity by 1 + percentage, and keep the price. 2. For "off", multiply the price by 1 - percentage, and keep the quantity. 3. Work out the price per unit for each deal. 4. State which price per unit is lower, with the two values.
Some questions give "3 for the price of 2" or "buy 4 packs and get the cheapest free". Work out the total cost of the whole bundle and divide by the number of items you receive. For "3 for the price of 2", you pay for 2 and get 3, so the cost per item is 2/3 of the normal price. This is a saving of one third, not of one half.
When a customer needs a fixed amount, such as at least 100 tiles, check every sensible combination of pack sizes. A special offer may only apply to one pack size or to a minimum number of packs. List the combinations that reach the target, work out each total cost, and compare. The cheapest combination may include a spare item. That is fine, because the question asks for the lowest cost, not the least waste.
Method: 1. Work out the cost and the number of items in each bundle. 2. List the combinations that reach the amount needed. 3. Apply any discount only where its condition is met. 4. Compare the total costs and name the cheapest combination.
Tip: Write the combinations in a small table with columns for packs, items and cost. It stops you missing one.
Command words such as "Which is the better value for money? You must show how you get your answer" give most of the marks for the working and the last mark for a clear conclusion. Show both unit costs, name the winner, and where possible say by how much. A line such as "Brand Y is cheaper at 153p per 100 g compared with 160p" is complete.
Real exam answers are marked on method, so even if a division is wrong a clear method scores. Do not round too early. If two unit prices are very close, such as 79.67p and 80p, rounding both to the nearest penny makes them look equal. Keep at least two decimal places, and only round the final value to the accuracy the question asks for.
Method: 1. Show the working for each deal as a clear division. 2. Keep at least two decimal places until the comparison is made. 3. Name the better deal and quote both unit costs. 4. If asked, say how much cheaper it is per unit or in total.
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