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Explanation:
Let x be the angle measure of that dark area. That angle x must add to the other angles 25, 90, and 25 to get a full 360 degree rotation.
So,
x+25+90+25 = 360
x+140 = 360
x = 360-140
x = 220
The dark area angle is 220 degrees
This is known as a reflex angle due to it being larger than 180 degrees, but it's less than 360 degrees. In symbolic terms, any reflex angle is one such that 180 < x < 360.