You need to know that the exterior angles of a polygon add up to 360 degs. So if it has 10 sides, it has 10 angles, and each ext angle is 360/10 = 36 degs.
Each int angle is 180 - 36 degs = 144 degs.
Or you can use the fact taht the sum of the interior angle = (n-2) x 180 degs, where n is the no of sides.