Steps of Construction:
1. Draw a line segment of any measure named as PQ.
2. With P as centre draw an angle of 120° with the help of protractor.
3. With P as centre again draw an arc which cuts line segments PQ and PS at M and N.
4. With M and N as centres and with radius little more than half the arc, draw arcs cutting each other at T. Join PT which is the required bisector of ∠SPQ = 120°.
∴ ∠TPQ = ∠TPS = 60° (each)