Father (80 kg) and son (30 kg) are sitting at one of the ends of a 4 m long boat (40 kg) standing still on water. They start to shift slowly. Father stopped at centre of boat but son stopped at other end. Neglecting friction with water, how far does the boat move on the water during the process?
(1) 1.87 m (2) 1.5 m (3) 1.25 m (4) 1.2 m