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Start on the right-hand side of the triangle:
-- The angle inside the triangle on the right end has to be the supplement
of the 115, because when you put them together, they form a straight line.
So that angle has to be (180 - 115) = 65 degrees.
-- Now you know 2 angles inside the triangle. Together, they add up to
(95 + 65) = 160 degrees.
-- You know that the angles inside a triangle always add up to 180 degrees,
and you already have 160 of them. So you know that the third angle, over
on the left side of the triangle, must be 20 degrees.
-- That skinny 20-degree angle, together with the 10x outside, are
supplementary, because when you put them together, they form a
straight line.
So ... 10x + 20 = 180
Subtract 20 from each side: 10x = 160
Divide each side by 10: x = 16 .
