Many many thanks Oyler. This was particularly well-timed. I've just started tutoring for the GMAT and this level of numerical reasoning is ideal for my students - and more fun than ploughing through questions.
*Digression* For those new to such things, the GMAT involves verbal and numerical reasoning and is one of the obstacles to getting into business school (should you wish to do so). If you're really at a loose end this week you can download free tests from
http://www.mba.com/mb...stpreparationsoftware
and see whether you'd get into Harvard's MBA programme (you'd need 720 - it's scored on a bell curve between 200 and 800). It's a multiple choice computer adaptive test and might help those who fear numericals brush up on high-level GCSE standard Maths. Also if anyone knows more than I about algorithms for such things and has the time, I'd love to pick your brains offline. Digression over.