Recall that the interior angles of a regular quadrilateral add up to 360 degrees, and a right angle has a measure of 90 degrees.
Let x° be the measure of the fourth angle, then we can set the following equation:
[tex]40^{\circ}+90^{\circ}+90^{\circ}+x^{\circ}=360^{\circ}.[/tex]Adding like terms we get:
[tex]220^{\circ}+x^{\circ}=360^{\circ}.[/tex]Subtracting 220 degrees from the above result we get:
[tex]\begin{gathered} 220^{\circ}+x^{\circ}-220^{\circ}=360^{\circ}-220^{\circ}, \\ x^\circ=140^\circ. \end{gathered}[/tex]Answer:
[tex]140^{\circ}.[/tex]