20 second problem

The sum of the exterior angles of a convex polygon is 360 degrees. The interior and exterior angles are complements (i. e., add to 180 degrees). Therefore the answer, in this case, is $180-\frac{360}{8}\Rightarrow 135$ .

There is a short cut formula for the interior angle of a regular polygon which is:

$\frac{(n-2)\times 180^\circ}{n}$ where $n$ is the number of sides of the regular polygon. Thus, the measure of the interior angle is: $\frac{6\times180^\circ}{8}=135^\circ$

The formula for finding the sum of all the angles that are inside is (n-2)×180. In this case n=8. so the sum of all the inside angles is 6×180=1080. Since there are 8 inside angles in total one inside angle is equal to 1080:8=135 degrees.