x and (4x - 20) are supplementary angles because they add up to 180°. You can tell this because the line which they are formed on is 180°.
Knowing that x + (4x - 20) = 180, you can solve for x:
x + (4x - 20) = 180
(x + 4x) - 20 = 180
5x - 20 = 180
5x = 200
x = 40°
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Answer: x = 40°