Inside the triangle one of the angle should be 50 (alternate angle )

Then 50+40 = 90

and 180-90 =90 so answer is 90. ]]>

What gets me is that before calculating I tried to think what I expected the answer to be, and came out with one-third, and I don’t know why that sounded right. I’m curious whether I had encountered the problem before and remembered the answer without remembering it, or whether there’s some obvious way to see it that my conscious mind isn’t noticing. Or if the lazy part of my mind figured the answer must be some nice simple fraction, and 1/2 is right out, and 1/4 looks too small, so 1/3 is the only plausible choice because this kind of setup never gives you a 2/5th or a 3/7th or something like that.

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