‘O’ is the centre; ∠POR = 120°
∠PQR = ∠POR [∵ angle made by an arc at the centre is, twice the angle made by it on the remaining part]
∠PSR = 1/2 [Angle made by \(\widehat {PQR}\) at the centre]
∠PSR = 1/2 [360° – 120°] from the fig
= 1/2 x 240 = 120°