Could you please explain the step by step process of setting up this problem? A 10 kg, 4 m long plank of wood is going to be used as a teeter-totter for a brother and sister. The brother has a mass of 30 kg and the sister a mass of 20 kg. If the brother and sister sit at opposite ends of the plank, how far from the brother should the fulcrum be in order for the teeter-totter to be balanced?
A. 1.33 m
B. 1.60 m
C. 1.67 m
D. 2 m