The sum of exterior angles of ANY regular polygon is 360.
For example, the exterior angle of a square is 90 degrees time 4 vertices = 360 degrees.
The exterior angles of an equilateral triangle is 180-60=120, times 3 vertices = 360.
...and so on.
So we can use this fact to calculate the exterior angle of a REGULAR decagon, which is therefore 360/10=36 degrees.
According to the diagram, this angle (36 degrees) is the complement of x, so x=90-36=54 degrees.
Next, the interior angle is just the supplement of the exterior angle, namely
y=180-36=144 degrees.
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