Sx T R 140 D

The angle subtended by the arc RT at the center S is 140 degrees. Since ST and SR are radii, triangle STR is isosceles. The angle at the circumference subtended by the same arc is half the angle at the center. Therefore, angle STR = 140/2 = 70 degrees. Since the line passing through T is tangent to the circle at T, the angle between the tangent and the radius ST is 90 degrees. Thus, angle RTD = 90 - angle STR = 90 - 70 = 20 degrees.