Question

The sum of seven interior angle measurements is an octagon equal 835°. What is the angle measurement of the eighth angle?

Answer 1

$\bf \textit{sum of all interior angles in a polygon}\\\\ S=180(n-2)~~ \begin{cases} n=\textit{number of sides}\\[-0.5em] \hrulefill\\ n=8 \end{cases}\implies S=180(8-2) \\\\\\ S=180(6)\implies S=1080$

now, we know 7 of those angles are already 835°, so the last one must be 1080 - 835 = 245°.