I didnt understand PLEASE HELP with the explanation please i need to understand to solve the rest

What is the missing reason in step 5? linear pair postulategivendefinition of complementary anglescongruent complements theorem

Find the nonpermissible replacement for binthis expression.b2/5b +5

Set $r$ is a set of rectangles such that (1) only the grid points shown here are used as vertices, (2) all sides are vertical or horizontal and (3) no two rectangles in the set are congruent. if $r$ contains the maximum possible number of rectangles given these conditions, what fraction of the rectangles in set $r$ are squares? express your answer as a common fraction.