Solution:
In Fig. if ∠ABC = 20º, then ∠AOC is equal to .
Given that,
. From figure we can see that, the angle made by an arc at the center is , and the angle made by arc at the circumference is .
Since the angle made by an arc at the center is two times the angle made by it at any point on the remaining part of the circle, so .
Therefore, required angle is .
