A section of highway is 18 km long. A bus going fromeast to west on this road climbs for 6 minutes at one speed, levels off for 6 minutes at a second speed and descends for 6 minutes at a third speed. Going from west to east on the same section of the road, the bus climbs for 12 minutes, levels off for 6 minutesand descends for 3 minutes. A passenger records the time forhalf of the trip going from west to east as 13 minutes. Whatare the speeds of the bus climbing, on the level, and descending?