A quadrilateral has three acute angles, each measure $80°$. What is the measure of the fourth angle? 

Let $\angle 1=\angle 2=\angle 3={80}^{\circ }, \angle 4=?$

We know that the sum of interior angles of quadrilaterals is always equal to $360°$

$\angle 1+\angle 2+\angle 3+\angle 4=360°$

$80°+80°+80°+\angle 4=360°$

$\angle 4=120°$

Therefore the fourth angle is $120°$