Question:
The three angles of a quadrilateral are respectively equal to 110°, 50° and 40°. Find its fourth angle.
Solution:
Let $x$ be the the fourth angle.
Since, the sum of all the angles of a quadrilateral is $360^{\circ}$, we have:
$110^{\circ}+50^{\circ}+40^{\circ}+x=360^{\circ}$
$\Rightarrow 200^{\circ}+x=360^{\circ}$
$\Rightarrow x=160^{\circ}$
$\therefore$ The fourth angle is $160^{\circ}$.