Triangle-50-60 Puzzle
Let ABS be an isosceles triangle with an angle of 20 degrees at S and 80 degrees at A and B.
Draw a line (blue) at an angle of 60 degree from A. It intersects the segment BS in U.
Draw a line (green) at an angle of 50 degree from B. It intersects the segment AS in V.
Determine the size of the angle BVU = α by elementary geometry means.
PS: Do not even think about putting it into a coordinate system or using trigonometric functions.