The total interior angles of a square (or rectangle) = 360°

Each time we add a side (triangle to square, square to pentagon, pentagon to hexagon), we add another 180°. Triangle to square $=180°+180°=360°$

1		2		3		4
$90°$	$+$	$90°$	$+$	$90°$	$+$	$90°$	$=$	$360°$

The total internal angles of a square = 360°