physics
posted by sony .
a boat of mass 90 kg is floating in still water a body of mass 30 kg walks from the stern to the boat the length of the boat is 3m. calculate the distance through which the boat move.

He doesn't move actually 3 meters, as the boat is moving a distance d beneath him. So his mass actually moves (3d).
The center of mass should remain at zero.
30(3d)+90*d=120*0
solve for d. Negative means in the direction opposite to his movement.