Question:
A quadrilateral has three acute angles each measures 80°. What is the measure of the fourth angle?
Solution:
Let $x$ be the fourth angle.
Since, the sum of all angles of a quadrilateral is $360^{\circ}$, we have:
$80^{\circ}+80^{\circ}+80^{\circ}+x=360^{\circ}$
$\Rightarrow 240^{\circ}+x=360^{\circ}$
$\Rightarrow x=120^{\circ}$
$\therefore$ The fourth angle is $120^{\circ}$.