Two people, one of mass 85 kg and the other of mass 50 kg, sit in a rowboat of mass 90 kg. With the boat initially at rest, the two people, who have been sitting at opposite ends of the boat, 3.5 m apart from each other, now exchange seats. How far does the boat move?

Answer 1

Here is the key. Center of gravity stays put. So two folks move, and between them, they move the cg, but the boat moves back that far. (Equal and opposite reaction)

so apparently 35 kg (85-50) is shifted 3.5m.

cg shift? 35/(90+85+50)*3.5 m

figure that out. Then, the boats shifts back that far.