A quadrilateral has three acute angles each measures $80°$. If the measure of the fourth angle is $x°$ then find $x$.

If the measure of $\angle MPN=x°$ then find $x$.

In the given figure, the bisectors of$\angle A$ and $\angle B$ meet at a point $P$. If $\angle C=100°$ and $\angle D=50°$, find the measure of $\angle APB$.