Official Guide Explanation:
Problem Solving #7

Background

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Solution and Metadata

Question: 7
Page: 62
Difficulty: 4 (Moderately Easy)
Category 1: Geometry > Lines >

Explanation: Since the diagonal line bisects angle ACD, we know that the two angles it creates are half the measure of the right angle. Since a right angle is 90 degrees, each of the angles are 45 degrees. The two angles formed by the diagonal line must sum to 180 degrees (this is true of any angles that combine to form a straight line), so combine x with one of the 45 degree angles formed by the diagonal:

180 = x + 45

x = 135, choice (E).

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