Three runners Adam, Ben, and Charles all start at the same time for a 24 km race, and each of them runs at a constant speed. When Adam finishes the race, Ben is 8 km behind, and Charles is 12 km behind. When Ben finishes the race, how far behind is Charles, in km?
When Adam finishes, the gap between Ben and Charles is 4 km. Also, the length of the race is 24 km, so the gap widens to 24/8*4 = 12 km by the end of the race. So Charles is behind by 12 km when Ben finishes.