.
It is 360° - (81° + 105° + 138°),
because the sum of four interior angles of any (convex) quadrilateral is equal to 360°.
Solved.
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To see other similar solved problems, look into the lesson
- Solved problems on angles of a triangle
- Solved problems on angles of a parallelogram
- Solved problems on angles of a quadrilateral
- Solved problems on angles of a polygon
- Solved problems on missed angle of a polygon
- Solved problems on angles of a regular polygon
in this site.