A maths teacher at Surbiton High School was about to enjoy a cheesy snack on Christmas Day when he saw something disturbing.

On the packaging of the Jacob’s Mini Cheddars, it stated “20 per cent extra free” had been added as there were 15 cheese biscuits packets instead of 12.

However, as Michael Gibbons has pointed out in an email to United Biscuits, which produces Mini Cheddars, the calculation was incorrect- there was actually 25 per cent extra free in the packet.

Your Local Guardian: The offending calculation 

Mr Gibbons, who has been teaching maths for 25 years- 12 of those at Surbiton High School- said he “didn’t want to be too cheesy about it” but “it’s vital our students are aware of maths in the real world”.

So he decided to highlight the inaccuracy to his class and the makers of the popular snack.

His email to the company read: “Dear United Biscuits,

“I write to express my ongoing concern over your multipack Mini Cheddars, which are baked with ‘real cheese’.

“On Christmas Day, I eagerly cracked open the bag you see in the photograph but had to put them down when I saw your claim on the front.

“You say that, in increasing the number of packets from 12 to 15, you are giving me ‘20 per cent’ extra free (3/12 × 100 rather than the 3/15 × 100 that I think you did when committing this mathematical crime).

“Luckily, I was able to rescue my Christmas Day and consoled myself by demolishing 100 per cent of the contents in rapid time.

“I work as a teacher of mathematics and showed your flawed wrapper to my Year 10. They were equally upset.

“I hope that you are able to rectify the error. Not just to cheer up snack loving mathematics teachers around the UK but also because you are doing yourselves a disservice.

“With very best wishes,

“Michael Gibbons.”

Mr Gibbons explained his motive to the Surrey Comet.

He said: “I want the students to question everything- there’s a lot of bad maths around.

“There’s a lot of maths that is deliberately used to scare you.”

The teacher gave an example of a national newspaper which ran a story that stated “you are 50 per cent more likely to get cancer if you eat red meat” than if you don’t.

In the study, two out of 1,000 people who avoided red meat got cancer, while three out of 1,000 people who ate red meat were diagnosed with the disease. 

Mr Gibbons said: “The figures aren’t wrong but they are very misleading.”

Although his class was “dismayed” by the mistake, he added: “My class was really sweet about it, they asked ‘No one’s going to get in trouble at United Biscuits are they?’”

United Biscuits has been contacted for comment.

Have you seen any mathematical inaccuracies recently? Contact: grainne.cuffe@newsquest.co.uk