The points A, B and C lie on a circle with centre O, as shown in the diagram - HSC - SSCE Mathematics Extension 1 - Question 12 - 2017 - Paper 1

Given that the angle ∠AOC is 100°, we can find the reflex angle ∠AOC:

$$ext{Reflex } ∠AOC = 360° - 100° = 260°.$$

Now, we use the Inscribed Angle Theorem, which states that the angle at the circumference (∠ABC) is half of the angle at the center (∠AOC):

∠ABC = rac{1}{2} imes ∠AOC = rac{1}{2} imes 100° = 50°.

Thus, the size of ∠ABC is 50°.