It took three hours from the foot of the mountain to the top of the mountain, and the average route was 900 meters per hour. Then they walked * * * 900 * 3 = 2700 meters up and down the mountain, it took 2700+2700 = 5400 meters to get up and down the mountain, and it took two hours to walk from the foot of the mountain to the top of the mountain, three hours and five hours, so their average speed up and down the mountain was 5400 meters.