This question is weird, because I asked some of my friend and my teacher to solve this and they couldn't answer or they had different answers (20° , 30° , 40° & lots of different number)
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2It is a famous problem, the answer is $30^\circ$. see Langley's adventitious angles – achille hui Sep 21 '21 at 06:07
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1There are many well known angle chasing problems in $80$-$80$-$20$ triangles. See here - https://www.cut-the-knot.org/triangle/80-80-20/IndexToClassical.shtml – Math Lover Sep 21 '21 at 06:09