An exam revision guide used by GCSE pupils has been withdrawn after 90 errors were found in it.
A team from Cardiff University’s School of Mathematics found that about every three in 50 questions contained an inaccuracy.
More than 1,000 copies of the guides have been destroyed and refunds have been offered to customers.
The revelation comes after The One Show commissioned Cardiff University to look at revision guides for exam boards WJEC, AQA, Pearson/Edexcel, SQA and OCR.
The WJEC guide, Mastering Mathematics for WJEC GCSE Practice Book: Higher, had the highest level of errors, with 90 mistakes out of 1,496 questions.
Dr Matthew Lettington, who oversaw the research, said the level of mistakes was “unacceptable” and some errors would have been “highly confusing” for pupils.
Hodder Education, the book’s publisher, apologised for the mistake.
Group managing director Lis Tribe said: “We are human. We do our best. We have made a mistake.
“Where our process fell down, which is a real concern to me and to my team, is that there wasn’t the final quality check that should have taken place. We simply missed a stage because of the pressure of getting the book out on time.
“We are actually very grateful to The One Show for bringing these errors to our attention and enabling us to withdraw the book and put them right.”
A spokesman for the WJEC said: “We have worked with Hodder to produce endorsed material. However, we have not endorsed the revision guide in question.
“As it has not been subjected to our endorsement process, we are not responsible for its content and cannot comment on it.
“Of course, if we are made aware of any errors in non-endorsed publications, we make every effort to ensure that the relevant publisher is informed.”