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This problem trains for: SAT-I, AMC-8, GMAT.

In terms of a° and b°, what is the measure of the angle x°?

This is a simple angle-chasing situation.

Focus on the angle that is the easiest to find.

In triangle BCD, use the fact that the angles of a triangle add up to 180°.

Use the same fact in triangle ABC:

and substitute the value for y: