Answer
The magnitude of the normal force of the roof on the worker is 799 N.
Work Step by Step
The magnitude of the normal force of the roof on the worker is equal to the component of the worker's weight which points perpendicular to the slope of the roof. So,
$F_N = mg~cos(\theta)$
$F_N = (850~N)~cos(20^{\circ})$
$F_N = 799~N$
The magnitude of the normal force of the roof on the worker is 799 N.
