But for some people it really is true - they just cannot do sums.

Campaigners for better awareness of dyscalculia - difficulty with numbers - reckon that about 420,000 pupils in England and Wales have the condition.

It is part of what is called the specific learning difficulty profile - which also includes dyslexia, which is probably better known and is better researched.

'We're not ill'

Jan Poustie had the problem herself and now lectures and writes about it.

Some children may be truly baffled by sums

She says that in some cases there has been a bleed into the brain, an identifiable physical problem.

"But people who have these conditions should not be regarded as being ill," she told The Learning Curve on BBC Radio 4.

"We are functioning people in a normal world, we just happen to have a difficulty in these areas."

Typical characteristics of dyscalculia include having difficulty with notions relating to time and direction - people may be late, notoriously. They may be bad at both day-to-day handling of money and longer-term financial affairs.

Mixed up

Numbers are often mixed up and results in simple sums may be inconsistent.

People can overcome their reading difficulties

All this is terribly familiar to Helen - not her real name, because even though she now has a successful career she worries what her employers might think if they knew.

"I'd always had problems with maths, but because I was in the top sets for English, I was automatically put in the top set for maths, where I floundered," she told News Online.

As exams approached, her school told her she could only sit a CSE - the old, lower grade of exam to O levels.

"It seemed fairly catastrophic at the time because I wanted to go to university," she recalls.

"My parents got me a private tutor, who suggested to them that I might have dyslexia. They thought that was daft because I did well at English and liked reading.

Revelation

"However, the tutor printed worksheets for me which had enlarged print, and used different coloured paper because that can make things easier for dyslexic students to read.

Dyscalculia need not be a barrier to successful work

"I got a grade one - equivalent to an O level - and thought no more of my difficulty with maths.

"A number of years later, in the course of my work, I visited a dyslexia institute. I was talking to the experts there generally about dyslexia, and came to realise that more and more of the indicators of the condition actually applied to me.

"They ran a few tests and talked to me about the things I had difficulty with. They told me about dyscalculia, which was quite a revelation to me.

Lottery numbers

"I really don't feel that I have to do a great deal about it, I get by fine and if I have anything I really need to know, I just ask for help.

"I can't tell the time, and I have a great deal of difficulty reading bank statements and shopping receipts.

"But there have always been ways of getting around those difficulties - friends tell me how much I've put onto my Visa card, and someone else always knows the time.

"Someone else always has to check my National Lottery numbers, because I just can't match the numbers on the screen with the ones in my hand.

"I've recently bought a very large analogue wall clock, and by covering some of the numbers with my hands, I can sometimes work out what the time must be by a process of elimination.

Getting lost

"I also have difficulty finding my way around - even within a building, and it takes me a long time to learn a route somewhere."

There are problems in grasping basic mathematical concepts

Teachers will identify such things in their numeracy work, Jan Poustie says.

"They can see these students are having problems with it. They may learn their tables but be unable to apply them, so they can't use them within sums.

"They can have problems with checking work, because they actually go down mental dead-ends. Because they have planning and organisational difficulties, they actually don't check reliably their work, they just go by intuition - you know: 'The answer feels right'."

Not careless

They can write numbers backwards. If they do one sum involving addition, they may use an addition sign in the next sum as well even though it involves subtraction.

"This is one of the big problems, where people are thinking they are being careless and they're not, they just become very confused."

So what is the best way for schools to treat pupils with dyscalculia when exams loom? Jan Poustie says it depends on the individual.

"We have the student who has a general learning difficulty, who is functioning below the average intellectual ability. They are going to have major problems across the board.

"For those students, certainly, we want them to be able to get competent with having normal life skills - especially budgeting and handling money.

"But a lot of students can actually cope with the higher forms of maths but aren't allowed to go on that far. They're stopped.