Bunuel wrote:

In the figure above, when ray OA is rotated clockwise 9 degrees about point O, then it will be perpendicular to ray OB. What is the measure of ∠AOB before rotation?

(A) 81°

(B) 89°

(C) 99°

(D) 93°

(E) 90°

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If ray OA is perpendicular to OB

after rotation, ∠AOB is

90 degrees.

If need be, draw the end result (in diagram, where OA is red) to track on ray's movement.

Original ray OA is rotated clockwise 9 degrees: it moves from left to right

9 degrees.

Measure of ∠AOB before rotation? It "shrank." So add: 90 + 9 = 99 degrees.

Answer C

You can do this problem with compass (absolute bearing and relative bearing) or clock angles; IMO, unnecessarily complicated. YMMV.

Edited for a typo that created a grammatical mistake which could cause problems in SC. Answer remains the same.
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