You have three hours for this exam (but part of it is impossible)

Every GCSE, A-level and AS-level exam paper is to be rigorously checked after it emerged that in the past week, all of the big three exam boards have made students sit tests containing questions that were impossible to answer.

The exams watchdog, Ofqual, described the mistakes as "unacceptable" and called on all boards to tighten up their checking procedures.

Glenys Stacey, the regulator's chief executive, told them in a letter: "We take instances like this very seriously. I am calling on awarding organisations to take steps now to protect students from further disruption and anxiety."

The first howler came in an AS-level maths paper sat by 6,790 teenagers, in which students were asked to find the shortest route to walk along a network of tracks in a forest using a mathematical equation. The Oxford, Cambridge and Royal Society of Art (OCR) exam board failed to calculate the length properly, however. It has promised to take the error into account when marking the exam. The question was worth eight marks, or 11 per cent of the paper.

In the second error, an AS-level business paper set by the Assessment and Qualifications Alliance (AQA) did not give enough information for pupils to work out the profits of a chocolate company. It was worth three marks.

Yesterday, it was revealed that a multiple-choice question in an Edexcel biology paper gave a selection of wrong answers, but not the correct one. It was worth only one mark. Students affected by the mistakes have raised concerns that they could affect their grades and in turn their university places. UCAS, the university admissions service, has already cautioned that up to 200,000 people may miss out on places this year due to a glut of applications from students trying to beat the rise in tuition fees scheduled for September 2012.

Dr Geoff Parks, the head of admissions at the University of Cambridge, said that AS-level results are often crucial in determining university admissions. They are the only post-GCSE exam results students can draw upon when making their applications.

In her letter to the exam boards, Ms Stacey added: "We expect, as I am sure you do, that all question papers should be free from error. I am aware that, where such errors do appear in question papers, awarding organisations are able to implement steps to limit the impact on the students' marks and grades.

"However, incidents of question errors can clearly disrupt students when taking an examination."

Dr Jim Sinclair, the director of the Joint Council for Qualifications, the umbrella body representing the exam boards, said: "Awarding bodies are aware that a small number of questions in this year's exams have contained errors and understand the distress this may have caused students.

"Students and parents should be assured that no one will be disadvantaged as a result of these mistakes. Examiners marking the papers are aware of the incidents and will make careful adjustments so that all students receive the marks they deserve."

Stumped? Don't panic...

If you are given a paper – whether it has any errors or not – it helps to bear these points in mind:

* If your mind goes blank, don't let it faze you. Just put your pen down and take a few deep breaths before starting again.

* Tell yourself that you are calm and that you will do well.

* Divide your exam time according to the marks allocated for each question.

* If you are stuck on a question, move to the next and come back to it later.

* Ask yourself: "Have I answered the question?", then move on.

* Check for factual mistakes, spelling and punctuation before you finish.