JAMB 2017 · UTME (set 2) · Q13

In the diagram, $R$ , $S$ and $T$ lie on a circle with centre $O$ . If $\angle RST = 60^\circ$ , find the angle marked $x$ .

Worked solution (try it first)

$x$ is at the centre and $\angle RST$ is at the circumference, both on arc $RT$ .
The angle at the centre is twice the angle at the circumference: $x = 2 \times 60^\circ = 120^\circ$ , option C.