Question

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.

Answer 1

He doesn't move actually 3 meters, as the boat is moving a distance d beneath him. So his mass actually moves (3-d).
The center of mass should remain at zero.

30(3-d)+90*d=120*0
solve for d. Negative means in the direction opposite to his movement.

Answer 2

-3/4m