Two ice skaters stand together as illustrated in the figures below. They "push off" and travel directly away from each other, the boy with a velocity of 0.500 m/s to the left. The boy weighs 800 N. Suppose you do not know the girl's mass but observed she moves at 0.640 m/s in the opposite direction. What is her mass (in kg)? (Consider the ice to be frictionless and the positive direction is to the right.)