An algebra problem by Clark Makmur
Andy, Brian, and Clark will be on a walk about 100 km long. Andy and Brian rode a motorcycle with average speed of 25 km/hour, while Clark walk with an average speed of 5 km/hour. After certain amount of distance, Brian went off the motorcycle and start to walk with an average speed of 5 km/hour, while Andy went back to pick up Clark and went to their initial destination. It is known, three of them arrive at the same time. How long was the trip ( in hours)?