Thanks all, I had worked out the colours part, but as usual, I was looking for something much more complicated. . I do it all the time! It was driving me to distraction!!
On a lighter note, and tongue in cheek, the maths "pedant" in me (remember Miss Marydoll's Maths Class thread) squirms at the use of the term sum. Sum means add The correct word is calculation , which we hammered into the weans (and teachers) on a daily basis.
By the way, what is the maths equivalent of a pedant? Suggestions on a postcard, please.
Thanks again, Far North.