Little Norma has mass 20kg and wants to use a 4.0-m board of mass 10kg as a seesaw. Her friends are busy, so Little Norma seesaws by herself by putting the support at the system's center of gravity when she sits on one end of the board. How far is she from the support point?

Answer 1

When they talk about the "system's" center of gravity, they mean the board and Norma together. The CG is x past the fulcrum (opposite Norma) and she sits 2-x meters from the fulcrum, with the fulcrum between her and the CG.

20*(2-x) = 10 x
40 - 20x = 10x
30x = 40
x = 4/3 meter