The question is a bit vague. I suspect the answer they're looking for is a variation of the way a motorcycle speedway GP works.
In a Speedway GP there are 16 riders and 4 starting gates. Each rider takes part in 5 heats, during which he races against each of the other riders once and starts from each gate at least once. This requires a total of 20 heats.
For the details of how this works see this scorecard (ignore the reserve riders 17 and 18).
So for 25 cyclists and 5 bicycles you need 30 heats, I'll leave the exact details of each heat as an exercise for the reader [img]/images/graemlins/smile.gif[/img]
Yup.. playing around with a definition of what constitutes a race, and what get measured during a race gives you a big leeway. And if you're even crazier, playing around of what constitutes a bike. (Let's say you have a 25 person bike with a power meter on each set of pedals, then you can measure most watts generated and reduce the problem to just 1 bike)