Statement: Complementary angles are two angles with measures that have a sum of 90. Reverse: Two angles with measures that have a sum of 90 are complementary angles.
Here the reverse is true, therefore, the statement is true biconditional.
Statement: A rectangle is a four-sided figure with at least one right angle. Reverse: A four sided figure with at least one right angle is a rectangle.
Here, the reverse is not true, therefore, the statement is not reversible.