Answer
The boy should sit .6 meters left of the pivot point.
Work Step by Step
We know that for the scale to read zero, the net torque must be 0. We know that the center of mass of the board is in the center, which is .4 meters right of the axis of rotation. Thus, we find how far away the boy should be on the left side:
$40r=(60)(.4)$
$r=.6 \ m$
