A large blue marble of mass 3.5 g is moving to the right with a velocity of 15 cm/s. The large marble hits a small red marble of mass 1.2 g that is moving to the right with a velocity of 3.5 cm/s. After the collision, the blue marble moves to the right with a velocity of 5.5 cm/s.

Use conservation of momentum: the total momentum before the collision equals the total momentum afterward. Let the red marble’s final velocity be v. Then 3.5(15) + 1.2(3.5) = 3.5(5.5) + 1.2v, with masses in grams and velocities in cm/s. Solving gives v = 37.45 ÷ 1.2 ≈ 31 cm/s, so the red marble’s velocity is 31 cm/s, choice D.

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