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I love that every comment focus on the math puzzle. Since the other stuff is clearly uninteresting.
It’s even easier than going the triangle route.
A four-corner shape always has 360° internally.
So the internal angle of corner X is 360-(60+40+35).
The exterior angle therefore is 360-360-(60+40+35) = 60+40+35 = 135
That’s based on the assumption that the two angles in the middle add up to 180, which can’t be assumed by inspection alone as demonstrated by the visibly square 80° angle
