He can work it out
Did you hear the tale of the king and the chessboard? The Indian King Shirham was so impressed with the new game that he offered its inventor, Sissa ben Dahir, a reward. But all Sissa asked was that a grain of wheat be placed on the first square, two grains on the second, four on the third, eight on the fourth, and so on until all the squares were full.
A modest enough request, thought the king, until a courtier worked out the total number of grains that would be required, which was more than the whole world could have produced in several years.
That's numbers for you: powerful and surprising yet, to most people, utterly terrifying.
Maths teacher Lawrence Potter learned as much when he stood in the car park of Tesco's in Brixton, south London, one day and asked shoppers if they wouldn't mind trying a spot of simple mental arithmetic. It was all in aid of a book he was writing, a book that would give grown-ups a second chance at a subject that had scared them since their schooldays.
"I wanted to find out the different techniques that people used to deal with numbers in real life," he says. But what he actually discovered was that most people's attitudes to maths have hardly changed since they were children. "One woman actually ran away when I asked her to do a division in her head." But if she ever reads Mr Potter's book, she will find on page 29 a charming and sincere apology for the distress its author caused her that day.
And read it she most certainly should. For Mathematics Minus Fear, published this week, succeeds very well in its intention: to do for numbers what Lynne Truss's Eats, Shoots and Leaves did for punctuation.
By starting at the very beginning - just how do civilisations manage without numbers? - and taking the development of maths in small, playful steps, he avoids the pitfalls of the traditional classroom approach many adults in the UK have experienced, which thunders like a train from simple addition to quadratic equations, and heaven help any child who falls off along the way. Such a child may never catch up. Much like Charlie, who sits in a corner at the back of every maths class and occupies his time creating minor distractions.
As well as leavening his narrative with 83 mathematical puzzles, of which the story of Sissa ben Dahir and his chessboard is but one example, Mr Potter weaves in the story of the fictional Charlie, providing timid readers with a sympathetic reference point should the going ever get rough.
And through Charlie we meet Mr Barton, the sad, bicycle-riding maths teacher who hovers, red Biro in clammy hand, ready to haunt the dreams of Charlies everywhere.
So is there a real-life Mr Barton, perhaps lurking at Graveney school in Streatham, where Lawrence Potter taught maths for two years after studying classics at Oxford? (The change of subject - maths seemed to offer more possibilities - involved teacher training at St Martin's, Lancaster, and then King's College London.) "No, no," he says. "The maths department at Streatham was actually full of interesting people, who made the job very enjoyable indeed." That said, Mr Potter thinks of himself primarily as a teacher, rather than a maths teacher. "I'm not someone who does maths problems before I go to bed or anything like that," he says. "But I do enjoy teaching."
It's a commitment that, in 2004, led him to sign up with Voluntary Service Overseas, and to accept a three-year placement at Gahini secondary school, a boarding school in Umutara province, eastern Rwanda. There, he teaches A-level maths (in between coaching the girls' football team), and finds that in rural Rwanda, just as in south London, the subject is regarded as daunting and difficult.
Despite this, he is enjoying the placement. "All people know about Rwanda outside the country is the genocide," he says. "But it's not something that touches my everyday life here. People at home think I must be in danger, but actually it is a safe place to be."
People at home also think that Mr Potter's book might make a rather good television series, and BBC Factual Entertainment is said to have shown interest in putting the 30-year-old teacher in front of the cameras, to take viewers on a mathematical journey stretching from ancient Babylon to the gambling halls of Las Vegas.
To which he simply says, "That would be good," before heading to the capital Kigali, two hours away, to play the German vice-consul at one more game of tennis (he has yet to beat him). His other aim is to perfect his recipe for the bean dish that is a staple in Rwandan restaurants. "The beans here are similar to kidney beans but multi-coloured, a bit like you imagine Jack's beans for the beanstalk. I figure that if I can perfect the recipe for making them soft and flavoursome then I have achieved some sort of integration into Rwanda."
Mathematics Minus Fear, by Lawrence Potter (Marion Boyars, pound;7.99 paperback)