Why the Formulas Work · Superchallenge problem

A vertical pole stands at one corner of a flat rectangular field that measures $30$ m by $40$ m. From the diagonally opposite corner of the field, the angle of elevation of the top of the pole is $30^\circ$. Find the angle of elevation of the top of the pole from the corner that is $30$ m from the pole, to the nearest tenth of a degree.