How far will the boat move problem, help please?

The center of mass of the system does not move, as there are no external forces. Let d be the distance of the center of mass from a fixed point lying somewhere on the boat's fore-aft axis. Let A and B be the distance of the 70kg and 60kg passenger's seat respectively from this point. Let C be the distance of the boat's center of mass from the point. From the definition of center of mass we have, before the swap:

d × (70 + 60 + 80) = 70A + 60B + 80C

After the swap, the boat, and its seats, have moved a distance ?x, and the passengers are in the opposite seats, and d does not change (no external forces), so we get

d × (70 + 60 + 80) = 60(A+?x) + 70(B+?x) + 80(C+?x)

Therefore

70A + 60B + 80C = 60(A+?x) + 70(B+?x) + 80(C+?x)

?x = (A - B)/21

But A - B is the distance between the seats 2.7m, so

?x = 2.7/21 = 0.129m

The boat moves 0.129m