It is an octagon so exterior = 360/8=45
interior =180-45=135
exterior =360/20=18
interior =180-18=162
The formula for interior angles in a regular polygon is (180(n-2))/n so I need to rearrange this so n is on the left:
(180(n-2))/n=176
180(n-2)=176n
180n-360=176n
4n=360
n=90
So there are 90 sides.
Hope this helps :)