A motorcyclist was sent by the Post Office to meet a plane at the airport. The motorcyclist knew exactly how much time he needed to reach the airport when the plane arrived, so he waited until the last possible minute to leave for the airport. However, one day, 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?
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