$\"\"$

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For a rectangle opposite triangles are congruent.

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$\"\"$ (CPCTC)

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$\"\"$ (Definition of congruent angles)

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$\"\"$ (Substitute $\"\"$ )

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$\"\"$ form a right angle.

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So, $\"\"$

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$\"\"$ (Substitute $\"\"$ )

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$\"\"$ (Subtract: $\"\"$ ) $\"\"$

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The measure of $\"\"$ is 60.