EditorTHE SUPERMARKET MATHS TRICK: GCSE skill that could stop shoppers falling for fake deals
With supermarket prices still high, many shoppers are trying to work out whether special offers actually save money – or just look good on the shelf.
From 50% off signs to buy-one-get-one-half-price deals, supermarkets are packed with percentage offers designed to tempt customers.
But many people struggle to work out whether those deals are actually worth it.
Consumer research has repeatedly highlighted how much prices can vary between retailers. In fact, our earlier report revealed how Aldi retained its title as the UK’s cheapest supermarket after a price comparison of major grocery chains.
Research also suggests around a third of adults feel nervous about maths, with some admitting it can even make them feel physically sick.
Now maths teacher Lyndsey Hartley says relearning a simple GCSE skill could help shoppers quickly check whether a deal is genuinely good – or just clever marketing.
“In my opinion, percentages are the gateway to becoming a bit of a money detective,” she said.
“They allow you to question your spending, decide whether something is a good deal or not, and get the most out of your money.”
The simple 10% trick
Hartley says the easiest way to tackle supermarket maths is to start by calculating 10% of a price.
“That’s because 10% can be broken down, or built up, to whatever you need,” she explained.
For example:
- 10% of £10 = £1
- 10% of £50 = £5
- 10% of £100 = £10
If the price isn’t a round number, simply move the decimal point one place.
- 10% of £15 = £1.50
- 10% of £55 = £5.50
Once you know 10%, other percentages become much easier.
- 5% = half of 10%
- 20% = double 10%
Spotting supermarket deals
The trick can quickly reveal whether offers are really worth it.
For example, if an £8 item is 25% off, start with 10%.
10% of £8 = 80p
Double that to get 20% (£1.60), then add half again to reach £2.
That means the final price should be £6.
Retailers frequently use promotions like buy-one-get-one-half-price deals, but the maths shows these offers are only worthwhile if shoppers actually need both items.
“If one item costs £6, the second at half price makes two items £9 in total – or £4.50 each,” Hartley explained.
“So it’s only really a saving if you planned to buy two anyway.”
Supermarkets have also introduced a range of initiatives aimed at helping shoppers through the cost-of-living crisis – including Asda’s £1 café meal deal for over-60s designed to provide cheaper meals for pensioners.
‘25% extra free’
Even promotions such as “25% extra free” can be checked quickly.
“If a pack has eight toilet rolls, 25% is a quarter – which means two extra,” Hartley said.
“So you’d expect ten in total.”
With households still facing rising bills – including water charges highlighted in our report on Welsh Water price increases affecting Swansea households – she believes shoppers should feel confident questioning deals.
The £1 rule
For those worried about getting the maths wrong, Hartley recommends a simple confidence trick.
“I like to use the £1 rule,” she said.
“If you tell yourself you only need to be accurate within £1, that instantly lowers the pressure massively.
“You’re not sitting an exam – you’re just sense-checking whether a deal is worth it.”
She hopes the approach will encourage people who feel anxious about maths to give it a try.
“Maths opens up doors to analytical thinking and problem solving,” she said.
“And when it comes to supermarket shopping, it can help you challenge deals and avoid wasting your money.”
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