A See-Saw Will Balance When the Pivot Is Placed at the Center

Multiple Choice

A see-saw will balance when the pivot is placed at the center of mass of the system.If a boy and his brother sit at distances 3L and L from the pivot,respectively,and the see-saw is balanced,what is the boy's mass if his brother's is 85 kg? Assume the see-saw board is uniform.

A) 28 kg
B) 64 kg
C) 21 kg
D) 57 kg