How do you calculate a best buy in GCSE Maths?

Divide each price by its corresponding quantity to find a common unit cost. The option with the lower cost per item, gram or millilitre is normally the best buy.

Should I divide price by quantity or quantity by price?

For unit cost, calculate $\frac{\text{price}}{\text{quantity}}$. Most importantly, use the same calculation and units for every option.

Can the more expensive pack be the best buy?

Yes. A pack with a higher total price may contain enough extra product to have a lower unit cost, making it better value.

When should I round a best buys calculation?

Keep the full calculator value while comparing options. Round only at the end, using an appropriate money value or the accuracy requested in the question.

Do I need to write a conclusion in a best buys question?

Yes. State which option is the best buy and support the choice using the comparable unit costs. This makes your decision clear to the examiner.

Common Best Buys Mistakes in GCSE Maths

Best buys questions can look almost too ordinary. Two packets, two prices, one choice. Yet many students lose marks because they compare the wrong numbers, overlook different units or round before the calculation is finished.

The common mistakes with best buys in GCSE Maths all come back to one principle: prices can only be compared fairly when the quantities are made equal. This usually means finding the cost of one item, one gram, one kilogram or another common quantity.

Imagine a student called Sam reviewing a marked paper. Sam understood what “best value” meant, but the working compared pack prices rather than equivalent quantities. The fix was not more complicated arithmetic. It was a more reliable method.

The best buys method

Use this checklist whenever you compare value for money:

Identify the price and quantity for each option.
Check whether the quantities use the same unit.
Choose one common comparison, such as the cost per item or cost per kilogram.
Calculate that value for every option.
Keep enough calculator digits until the final comparison.
State which option is the best buy and support the decision with figures.

The central calculation is:

cost=pricequantity\text{unit cost}=\frac{\text{price}}{\text{quantity}}

The lowest unit cost is normally the best buy, provided the question asks only about value for money.

A student treating supermarket best buys like a detective case

Mistake: choosing the lowest pack price

Sam first sees these cereal offers:

Pack A contains $g500\text{ g}$ and costs $£ 2.40$ .
Pack B contains $g750\text{ g}$ and costs $£ 3.15$ .

Pack A is cheaper to buy, but that does not prove it is better value. The packs contain different amounts.

Correct method

Find the cost per gram.

For Pack A:

£2.40 \div 500=£0.0048

For Pack B:

£3.15 \div 750=£0.0042

Pack B has the lower cost per gram, so Pack B is the best buy.

You could instead compare the cost per $g100\text{ g}$ :

£0.0048×100=£0.48£0.0048\times100=£0.48

£0.0042×100=£0.42£0.0042\times100=£0.42

Both approaches give the same conclusion. Cost per $g100\text{ g}$ can simply be easier to interpret.

Mistake: dividing in different directions

A common best buys mistake is calculating price divided by quantity for one product, then quantity divided by price for the other. These answers represent different things and cannot be compared.

Suppose one box contains $12$ bars for $£ 3.30$ and another contains $15$ bars for $£ 3.90$ .

Correct method

Use price divided by number of bars both times.

First box:

£3.30÷12=£0.275£3.30\div12=£0.275

Second box:

£3.90÷15=£0.26£3.90\div15=£0.26

The second box is the best buy because each bar costs $£ 0.26$ , compared with $£ 0.275$ in the first box.

Writing the calculation as a fraction helps keep the order consistent:

items\frac{\text{price}}{\text{number of items}}

Once you have this method in hand, use the Maths Genie Lessons and Question Bank to practise setting up the comparison correctly rather than relying on which pack merely looks cheaper.

Mistake: comparing different units

Sam then meets a question in which one quantity is given in kilograms and another in grams. Dividing immediately would produce unit costs based on different units.

A $kg1.5\text{ kg}$ bag of rice costs $£ 4.20$ . A $g900\text{ g}$ bag costs $£ 2.70$ .

Correct method

Convert the quantities to the same unit first.

g1.5\text{ kg}=1500\text{ g}

For the larger bag:

gram£4.20\div1500=£0.0028\text{ per gram}

For the smaller bag:

gram£2.70\div900=£0.003\text{ per gram}

The $kg1.5\text{ kg}$ bag is the best buy because $£ 0.0028 < £ 0.003$ per gram.

Useful metric facts include:

g1\text{ kg}=1000\text{ g}

ml1\text{ litre}=1000\text{ ml}

Converting first makes the comparison fair and reduces the chance of attaching the wrong unit to an answer.

Mistake: rounding too early

Some best buys GCSE questions use values that produce long decimals. Rounding those values too soon can hide a small but important difference.

Consider these two offers:

Offer A: $18$ bottles for $£ 11.50$ .
Offer B: $24$ bottles for $£ 15.20$ .

For Offer A:

£11.50÷18=£0.638888…£11.50\div18=£0.638888\ldots

For Offer B:

£15.20÷24=£0.633333…£15.20\div24=£0.633333\ldots

If both results were carelessly rounded to $£ 0.64$ at an early stage, the offers might appear equal. Keeping the calculator values shows that Offer B has the lower unit cost.

Round money appropriately only after the comparison, unless the question asks for a specified degree of accuracy.

Mistake: ignoring special offers

A discount changes either the price paid or the quantity received. It must be included before the unit cost is calculated.

A shop sells packs of $8$ yoghurts for $£ 3.60$ with a “buy two packs, get one pack free” offer. Another shop sells $20$ yoghurts for $£ 5.80$ .

Correct method

Under the special offer, paying for two packs costs:

2×£3.60=£7.202\times£3.60=£7.20

Three packs contain:

yoghurts3\times8=24\text{ yoghurts}

The offer's unit cost is:

£7.20÷24=£0.30£7.20\div24=£0.30

For the other shop:

£5.80÷20=£0.29£5.80\div20=£0.29

The second shop is the best buy. The word “free” is persuasive, but the complete calculation matters more than the label.

Mistake: giving a choice without evidence

A correct numerical comparison may earn method marks, but a response should still answer the question directly. Do not finish with two unexplained decimals.

A strong conclusion names the product and includes the comparison:

The second shop is the best buy because its cost is $£ 0.29$ per yoghurt, which is less than $£ 0.30$ per yoghurt under the first shop's offer.

This final sentence makes the reasoning visible. It also gives you a quick opportunity to spot whether you accidentally selected the larger unit price.

A reliable exam routine

Sam's corrected routine is short enough to use under exam pressure:

Make the units match

Convert kilograms to grams, litres to millilitres or otherwise put both quantities into an equivalent form.

Choose one comparison

Use cost per item, cost per gram or cost for a common quantity. Apply the same comparison to every option.

Keep calculator accuracy

Do not round working values before deciding which cost is lower.

Write the conclusion

Name the best buy and quote the comparable figures with units.

The arithmetic in these questions is often accessible. The real test is whether the comparison is consistent.

Exam-style practice

Try these without looking back at the worked examples.

Question A

A $g600\text{ g}$ packet costs $£ 2.64$ . A $g900\text{ g}$ packet costs $£ 3.87$ . Determine the best buy.

Question B

A pack of $10$ pens costs $£ 4.25$ . A pack of $16$ pens costs $£ 6.56$ . Determine the best buy.

Question C

A $litre2\text{ litre}$ bottle costs $£ 3.50$ . A $ml750\text{ ml}$ bottle costs $£ 1.35$ . Determine the best buy.

Answers

For Question A:

£2.64÷600=£0.0044£2.64\div600=£0.0044

£3.87÷900=£0.0043£3.87\div900=£0.0043

The $g900\text{ g}$ packet is the best buy.

For Question B:

£4.25÷10=£0.425£4.25\div10=£0.425

£6.56÷16=£0.41£6.56\div16=£0.41

The pack of $16$ pens is the best buy.

For Question C, first use $ml2\text{ litres}=2000\text{ ml}$ :

£3.50÷2000=£0.00175£3.50\div2000=£0.00175

£1.35÷750=£0.0018£1.35\div750=£0.0018

The $litre2\text{ litre}$ bottle is the best buy.

Turn the correction into marks

Sam's progress came from replacing guesswork with one repeatable comparison: match the units, calculate the same unit cost, retain accuracy and justify the choice. Knowing that routine is useful, but marks depend on being able to apply it when quantities, units and offers change.

Build that habit with the Maths Genie Lessons and Question Bank, focusing on best buys questions until the setup feels automatic.

The best buys method

Use this checklist whenever you compare value for money:

Identify the price and quantity for each option.
Check whether the quantities use the same unit.
Choose one common comparison, such as the cost per item or cost per kilogram.
Calculate that value for every option.
Keep enough calculator digits until the final comparison.
State which option is the best buy and support the decision with figures.

The central calculation is:

cost=pricequantity\text{unit cost}=\frac{\text{price}}{\text{quantity}}

The lowest unit cost is normally the best buy, provided the question asks only about value for money.

A student treating supermarket best buys like a detective case

Mistake: choosing the lowest pack price

Sam first sees these cereal offers:

Pack A contains $g500\text{ g}$ and costs $£ 2.40$ .
Pack B contains $g750\text{ g}$ and costs $£ 3.15$ .

Pack A is cheaper to buy, but that does not prove it is better value. The packs contain different amounts.

Correct method

Find the cost per gram.

For Pack A:

£2.40 \div 500=£0.0048

For Pack B:

£3.15 \div 750=£0.0042

Pack B has the lower cost per gram, so Pack B is the best buy.

You could instead compare the cost per $g100\text{ g}$ :

£0.0048×100=£0.48£0.0048\times100=£0.48

£0.0042×100=£0.42£0.0042\times100=£0.42

Both approaches give the same conclusion. Cost per $g100\text{ g}$ can simply be easier to interpret.

Mistake: dividing in different directions

A common best buys mistake is calculating price divided by quantity for one product, then quantity divided by price for the other. These answers represent different things and cannot be compared.

Suppose one box contains $12$ bars for $£ 3.30$ and another contains $15$ bars for $£ 3.90$ .

Correct method

Use price divided by number of bars both times.

First box:

£3.30÷12=£0.275£3.30\div12=£0.275

Second box:

£3.90÷15=£0.26£3.90\div15=£0.26

The second box is the best buy because each bar costs $£ 0.26$ , compared with $£ 0.275$ in the first box.

Writing the calculation as a fraction helps keep the order consistent:

items\frac{\text{price}}{\text{number of items}}

Once you have this method in hand, use the Maths Genie Lessons and Question Bank to practise setting up the comparison correctly rather than relying on which pack merely looks cheaper.

Mistake: comparing different units

Sam then meets a question in which one quantity is given in kilograms and another in grams. Dividing immediately would produce unit costs based on different units.

A $kg1.5\text{ kg}$ bag of rice costs $£ 4.20$ . A $g900\text{ g}$ bag costs $£ 2.70$ .

Correct method

Convert the quantities to the same unit first.

g1.5\text{ kg}=1500\text{ g}

For the larger bag:

gram£4.20\div1500=£0.0028\text{ per gram}

For the smaller bag:

gram£2.70\div900=£0.003\text{ per gram}

The $kg1.5\text{ kg}$ bag is the best buy because $£ 0.0028 < £ 0.003$ per gram.

Useful metric facts include:

g1\text{ kg}=1000\text{ g}

ml1\text{ litre}=1000\text{ ml}

Converting first makes the comparison fair and reduces the chance of attaching the wrong unit to an answer.

Mistake: rounding too early

Some best buys GCSE questions use values that produce long decimals. Rounding those values too soon can hide a small but important difference.

Consider these two offers:

Offer A: $18$ bottles for $£ 11.50$ .
Offer B: $24$ bottles for $£ 15.20$ .

For Offer A:

£11.50÷18=£0.638888…£11.50\div18=£0.638888\ldots

For Offer B:

£15.20÷24=£0.633333…£15.20\div24=£0.633333\ldots

If both results were carelessly rounded to $£ 0.64$ at an early stage, the offers might appear equal. Keeping the calculator values shows that Offer B has the lower unit cost.

Round money appropriately only after the comparison, unless the question asks for a specified degree of accuracy.

Mistake: ignoring special offers

A discount changes either the price paid or the quantity received. It must be included before the unit cost is calculated.

A shop sells packs of $8$ yoghurts for $£ 3.60$ with a “buy two packs, get one pack free” offer. Another shop sells $20$ yoghurts for $£ 5.80$ .

Correct method

Under the special offer, paying for two packs costs:

2×£3.60=£7.202\times£3.60=£7.20

Three packs contain:

yoghurts3\times8=24\text{ yoghurts}

The offer's unit cost is:

£7.20÷24=£0.30£7.20\div24=£0.30

For the other shop:

£5.80÷20=£0.29£5.80\div20=£0.29

The second shop is the best buy. The word “free” is persuasive, but the complete calculation matters more than the label.

Mistake: giving a choice without evidence

A correct numerical comparison may earn method marks, but a response should still answer the question directly. Do not finish with two unexplained decimals.

A strong conclusion names the product and includes the comparison:

The second shop is the best buy because its cost is $£ 0.29$ per yoghurt, which is less than $£ 0.30$ per yoghurt under the first shop's offer.

This final sentence makes the reasoning visible. It also gives you a quick opportunity to spot whether you accidentally selected the larger unit price.

A reliable exam routine

Sam's corrected routine is short enough to use under exam pressure:

Make the units match

Convert kilograms to grams, litres to millilitres or otherwise put both quantities into an equivalent form.

Choose one comparison

Use cost per item, cost per gram or cost for a common quantity. Apply the same comparison to every option.

Keep calculator accuracy

Do not round working values before deciding which cost is lower.

Write the conclusion

Name the best buy and quote the comparable figures with units.

The arithmetic in these questions is often accessible. The real test is whether the comparison is consistent.

Exam-style practice

Try these without looking back at the worked examples.

Question A

A $g600\text{ g}$ packet costs $£ 2.64$ . A $g900\text{ g}$ packet costs $£ 3.87$ . Determine the best buy.

Question B

A pack of $10$ pens costs $£ 4.25$ . A pack of $16$ pens costs $£ 6.56$ . Determine the best buy.

Question C

A $litre2\text{ litre}$ bottle costs $£ 3.50$ . A $ml750\text{ ml}$ bottle costs $£ 1.35$ . Determine the best buy.

Answers

For Question A:

£2.64÷600=£0.0044£2.64\div600=£0.0044

£3.87÷900=£0.0043£3.87\div900=£0.0043

The $g900\text{ g}$ packet is the best buy.

For Question B:

£4.25÷10=£0.425£4.25\div10=£0.425

£6.56÷16=£0.41£6.56\div16=£0.41

The pack of $16$ pens is the best buy.

For Question C, first use $ml2\text{ litres}=2000\text{ ml}$ :

£3.50÷2000=£0.00175£3.50\div2000=£0.00175

£1.35÷750=£0.0018£1.35\div750=£0.0018

The $litre2\text{ litre}$ bottle is the best buy.

Turn the correction into marks

Build that habit with the Maths Genie Lessons and Question Bank, focusing on best buys questions until the setup feels automatic.

The best buys method

Mistake: choosing the lowest pack price

Correct method

Mistake: dividing in different directions

Correct method

Mistake: comparing different units

Correct method

Mistake: rounding too early

Mistake: ignoring special offers

Correct method

Mistake: giving a choice without evidence

A reliable exam routine

Make the units match

Choose one comparison

Keep calculator accuracy

Write the conclusion

Exam-style practice

Question A

Question B

Question C

Answers

Turn the correction into marks

Frequently Asked Questions

How do you calculate a best buy in GCSE Maths?

How do you calculate a best buy in GCSE Maths?

Should I divide price by quantity or quantity by price?

Should I divide price by quantity or quantity by price?

Can the more expensive pack be the best buy?

Can the more expensive pack be the best buy?

When should I round a best buys calculation?

When should I round a best buys calculation?

Do I need to write a conclusion in a best buys question?

Do I need to write a conclusion in a best buys question?

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The best buys method

Mistake: choosing the lowest pack price

Correct method

Mistake: dividing in different directions

Correct method

Mistake: comparing different units

Correct method

Mistake: rounding too early

Mistake: ignoring special offers

Correct method

Mistake: giving a choice without evidence

A reliable exam routine

Make the units match

Choose one comparison

Keep calculator accuracy

Write the conclusion

Exam-style practice

Question A

Question B

Question C

Answers

Turn the correction into marks

Frequently Asked Questions

How do you calculate a best buy in GCSE Maths?

How do you calculate a best buy in GCSE Maths?

Should I divide price by quantity or quantity by price?

Ready to breeze through your GCSEs?

Ready to breeze through your GCSEs?