A is the position of Ankur.
B is the position of Syed.
C is the position of David.
In the equilateral triangle ABC, altitude, median, and angle bisector are all same.
⇒OB bisects ∠B [∵ OB is the angle bisector]
⇒∠OBD=60∘2=30∘
In ΔOBD,cos30∘=BDOB=a2R [∵R=20 m= radius of the circle]
⇒a=2Rcos30∘
⇒a=2×20×√32=20√3m [∵cos30∘=√32 ]
Hence, the length of the string is 20√3 m