The Mail Plane (riddle)

[Ace Mercury]

Originally Posted by http://www.futilitycloset.com/2009/11/19/the-mail-plane/

A motorcyclist was sent by the post office to meet a plane at the airport.

The plane landed ahead of schedule, and its mail was taken toward the post office by horse. After half an hour the horseman met the motorcyclist on the road and gave him the mail.

The motorcyclist returned to the post office 20 minutes before he was expected.

How many minutes early did the plane land?

Spread this at work on Friday, sort of disabled three other people that morning while we were working on figuring out the answer. We ended up coming up with three different methods of solution, but the same answer, which was rather gratifying.

Here's some assumptions that you sort of need to make in order to get the "official" answer. Anyone who's worked with riddles before can probably enumerate these based on cliches of mathy riddles.:
Spoiler Below

The mail transfer between the airplane and the horse and between the horse and the motorcycle is instantaneous. The horse and the motorcycle travel at their respective constant speeds and use the same route between the airport and the post office. The horse leaves the airport towards the post office the instant the plane arrives. The motorcycle leaves the post office at a time such that it would arrive at the airport exactly at the expected plane arrival time.

[Deku Trii]

Spoiler Below

I'm thinking 40 minutes. By not going 2 times the distance between the meet-up spot and the airport, the motorcyclist saved 20 minutes on his trip. The motorcycle was to be at the airport, then, 10 minutes after the moment where the motorcyclist and the horseman met up. The plane had already been at the airport for 30 minutes at the time of the meet-up, and was supposed to arrive 10 minutes later than that, so it was 40 minutes early.