Draw an angle of 80° with the help of a protractor and bisect it.

Steps of construction

1. Draw $\angle \mathrm{AOB}=80^{\circ}$ using protractor.

2. With O as centre and a convenient radius, draw an arc cutting AO at N and OB at M. 
3. With N as centre and a convenient radius, draw an arc. 
4. Similarly, with M as centre and same radius, cut the previous drawn arc and name it as point C. 
5. Join OC.
OC is the required angle bisector. 
On measuring we get

$\angle \mathrm{AOC}=\angle \mathrm{BOC}=40^{\circ}$