Ric, mass 117.3 kg, and Sharon, who is lighter, are enjoying Lake Snowden at dusk in a 45.0 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. Ric notices that the canoe moved 31.1 cm relative to a submerged log during the exchange and calculates Sharon's mass, which she has not told him. What is it?