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.