Agirl of mass mg is standing on a plank of mass mp. both are originally at rest on a frozen lake that constitutes a frictionless, flat surface. the girl begins to walk along the plank at a constant velocity vgp to the right relative to the plank. (the subscript gp denotes the girl relative to plank. use any variable or symbol stated above as necessary.)

1.) what is the velocity vpi of the plank relative to the surface of the ice?

2.) what is the girl's velocity vgi relative to the ice surface?

any understanding this would be appreciated.