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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
No, you should completely ignore the bottom half of the center line. You end up with a shape with four turns. Those four internal angles always add to 360.
He is saying your shape might have 5 turns instead of 4 if the bottom line is not actually a line. Though if that was the case, the problem would not be solvable, so I doubt it.
Nice one, forgot this was an option too. You are missing a set of brackets though ;)