Guidelines for building a statistical model for a business problem?

Hi There,

I would need some guidelines for creating a statistical, probabilistic model for a business problem I'm dealing with. Here is the problem:

I have a website that connects tourists to tour guides. When a new inquiry comes, a tour guide is assigned (today quasi randomly) to the inquiry. The tour guide creates a proposal and the potential guest decides if he/she takes the tour or not. By now I have some statistics on each guide, for example, a guide called Peter was assigned to 20 inquiries and he got positive feedback (tour ordered) from 15 of those. So his success rate is 75%.

My question is: how reliable this 75% is? After how many inquiries can I say with at least 90% probability that Peter's success rate is 75% plus/minus 5%?

What I need is some guidelines on how to model this problem and how to use the model to answer the question in my example above. I learned some basic statistics in school, so I think having the right starting point I could hopefully work out the model on my own.

Many thanks,