Finding Carmelita's mass from canoe displacement during seat exchange

[suspenc3]

Ricardoof mass 80Kg, and Carmelita, who is lighter, are enjoying Lake Merced at dusk in a 30Kg canoe. When the canoe is at rest in the placid water, they exchange seats, which are 3m apart and symmetrically located with respect to the canoe's centre. Ricardo notices that the canoe moves 40 cm relative to a submerged log during the exchange and calculates Carmelita's mass, which she has not told him. What is it?

I'm guessing that I should first find The COM of the canoe with the people in it. How can I do this with an unknown second mass?

 

[dx]

Lets say the unknown mass is M. write the expression for the center of mass with this variable. Write the expression for the center of mass in the new position. You know that they will be in the same place.