The answer is: [C]: 150° .
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Supplementary angles, by definition, add up to 180° .
So given the measure of one angle is 30° ; the measure of its supplement angle; PLUS 30°; must equal 180° .
So, x + 30 = 180 ; Solve for "x" .
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180 − 30 = x ; x = 150 .
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Its supplement angle measures: 150° ; which is answer choice: [C] .
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