A decagon is a polygon having $10$ sides. What is the sum of all the internal angles of a decagon?

In the given figure, $PQRS$ is a parallelogram with $\angle SPQ=80°$ and $\angle SQR=45°$. Find the measure of $\angle RSQ$ and $\angle PSQ$.

In the given figure, $ABCD$ is a parallelogram with $\angle A=70°$. The bisectors of $\angle B$ and $\angle C$ meet at $P$ inside the parallelogram. Find the measure of $\angle BPC$.

In the given figure, $PQRS$ is a parallelogram. Find the value of $\angle x, \angle y,$ and $\angle z$.