Is there a way to do these problems without resorting to plugging in?

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7

<p>Angle BCD=90-y
Angle ABC=(180-x)/2
Because AB//CD, Angle BCD=Angle ABC
or,90-y=(180-x)/2
Solving u get y=x/2
Therefore the answer is D</p>

<p>However I would like to know how plugging values will work here.</p>