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The momentum conservation is used to solve this question.

A 45.5 kg girl is standing on a plank that has a mass of 143 kg. The plank, originally at rest, is free to slide on a frozen lake, which is a flat, frictionless supporting surface. The girl begins to walk along the plank at a constant speed of 1.41 m/s relative to the plank.

(a) What is her speed relative to the ice surface?

Your answer differs from the correct answer by 10% to 100%. m/s
(b) What is the speed of the plank relative to the ice surface?