I am stuck on a problem which goes as follows: A sledge of mass 120kg stands at rest on the horizontal surface of an icy lake. A man of mass 70kg stands at one end of the sledge so that initially the distance from the man to the shore is 20m. The man now walks 3m relative to the sledge, towards the shore. Then he stops. Assuming that the sledge movies without friction on the ice, how far from the shore is the man when he stops? Any guidance would be helpful. I don't need the answer, just a general way to go about finding distance in terms of mass and conservation of momentum.