Help- Center of Mass/system of particles problem

[ldh5056]

Hoping someone can help me with this problem:

Ricardo, mass 80 kg, and Carmelita, who is lighter, are enjoying Lake Merced at dusk in a 25 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?

Thank you so much!

 

[Yuqing]

Sorry mis-phrased it. I mean to the say that the center of mass in a closed system does not change from internal actions. What you need to do is write two equations describing the location of the center of the mass for the initial and final states and equate them.