Can anyone explain to me how to do the following problem? Ricardo, mass 75 kg, and Carmelita, who is lighter, are enjoying Lake Merced at dusk in a 20 kg canoe. When the canoe is at rest in the placid water, they exchange seats, which are 3.0 m apart and symmetrically located with respect to the canoe's center. Ricardo notices that the canoe moved 50 cm relative to a submerged log during the exchange and calculates Carmelita's mass, which she has not told him. What is it? Thanks!