A boat went down the river for a distance of 20 km. It then turned back and returned to its starting point, having travelled a total of 7 hours. On its return trip, at a distance of 12 km from the starting point it encountered a log which had passed the starting point at the moment at which the boat had started downstream. The downstream speed of the boat is 1) 10.5 km/h 2) 12 km/h 3) 8 km/h 4) 10 km/h 5) None of these