Steps of Construction
1. Draw an angle AOC = 40° with a protractor.
2. Taking O as centre and some radius, draw an arc of a circle, which intersects OA at P and OB at Q.
3. Taking P as centre and radius QP. draw an arc of a circle, which intersects the arc drawn in step 2, say at a point R.
4. Join OQ and produce to form a ray OC. Then, ∠COB = 80°