Since,
PQ is diameter
Then,
∠PRQ = 90°
(Angle in semi-circle)
Therefore,
∠PRQ + ∠TRQ = 180°
(Linear pair)
90° + ∠TRQ = 180°
∠TRQ = 90°
By degree measure theorem,
∠ROS = 2∠RQS
∠RQS = 20°
In triangle RQT, we have
∠RQT + ∠QRT + ∠RTS = 180°
(By angle sum property)
20° + 90° + ∠RTS = 180°
∠RTS = 70°