Angles X and Y are supplementary. Angle X is 3 times the measure of angle Y. What is the measure of angle X?

Supplementary angles add up to 180°. Since angle X is 3 times angle Y, the four equal parts (three for X and one for Y) total 180°, so each part is 45° and angle X is 3 × 45° = 135°. The correct choice is D, 135°.

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