A `45.0kg` girl is standing on plank that has a mass of `150kg`. 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 alnong the plank at a constant speed of `1.50m//s` relative to the plank. (a) What is her speed relative to the ice surface? (b) What is the speed of the plank relative to the ice surface?