Ricardo, mass 80 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 45 cm relative to a submerged log during the exchange and calculates Carmelita's mass, which she has not told him. What is it? I don't really get those sort of problems, so I tried cutting and pasting into a problem similar to this one. This is what I tried to do: xcom = xcom 80(0) + 20(1.5) + 3m = 80(3-x) + 20(1.5-x) + m(3-x) After plugging 0.45 for x, I ended up with m = 500 kg. What's the right way to do this problem?