Question

A uniform wooden plank of length L = 6.0 m and mass M = 90 kg rests on top of two sawhorses separated by D = 1.5 m, located equal distances from the center of the plank. You try to stand on the right-hand end of the plank. If the plank is to remain at rest, how massive can you be?

Answer 1

90 kg at 1.5 / 2 meters from center of plank exactly balanced your mass m at 3 meters from center
90 * 1.5 / 2 = m * 3
45 * 1.5 = 3 m
m = 22.5 kg

Answer 2

m = M x D / L - D
m = 90 x 1.5 / 6 - 1.5
m = 30 Kg