\"\"

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The measure of an interior angle is 60.

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Sum of the measures of interior angles is (60)n.

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The sum of the measures of interior angles of a polyon is \"\".

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\"\"                     (Interior angle sum theorem)

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\"\"              (Substitute \"\")

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\"\"                 (Distributive property)

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\"\"            (Subtract 60n from each side)

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

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\"\"                           (Add 360 to each side)

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\"\"                                     (Divide each side by 120)\"\"

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The number of side in the polygon is 3.