On an ice skating rink, a girl of mass 50 kg stands stationary, face to face with a boy of mass 80 kg. The children push off of one another, and the boy moves away with a velocity of +3 m/s.

The children start at rest, so their total momentum is zero. After they push off, momentum is conserved: (50 kg)(v_girl) + (80 kg)(+3 m/s) = 0. Solving gives v_girl = −240/50 = −4.8 m/s, so choice C, −4.8 m/s, is correct; the negative sign means she moves opposite the boy.

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