This is basically a torque problem.You have to draw a diagram!

The board is pivoted at the shoulder. The force of gravity is considered to be acting at the geometric center of the board, a little behind the shoulder. His arm exerts a force 0.2 m in front of the shoulder. Our objective here is to maintain the resting state of the board.

Therefore:

Where the torque produced by the gravity force is counterclockwise and the torque produced by the force exerted by the arm is clockwise. Note here that the direction of the forces will be assumed to be perpendicular.

Where d is the distance from pivot. Note that distance of the gravity force from pivot is:

Rearrange the equation: