How many buses are there?

Woman: I think we’ve got the transportation plan worked out. We have four buses to take people from the hotel to the convention center. That should be enough, don’t you think?
Man: Yes, I think so. It takes five minutes to load one bus and ten minutes to make the trip, so we could plan to have one bus leave every fifteen minutes.
Woman: That sounds like a good schedule. Each bus could make the round trip in thirty minutes, and no one would have to wait too long for a ride. Then we won’t have to worry about anybody’s ruffled feelings.