Why do we interpret this vertical angle to be a horizontal angle?

[CivilSigma]

At the point B, there is supposed to be a force B that is perpendicular to the angled piece of metal. Why is the angle that the force makes with the horizontal 20? Should it not be 70?

 

[haruspex]

The metal makes an angle 20 to the vertical, yes? So what angle does the force make to the vertical?