QS is a diameter, which makes angle SPQ to be 180 degrees (ie a straight angle). Arc QTS is also going to be 180 degrees, because all semicircles are half of the full circle of 360 degrees. So (1/2)*360 = 180.
Angle QPR is a central angle that subtends (cuts off) the minor arc RQ, which is shown to be 50 degrees. The central angle is equal to this arc measure, so angle QPR is also 50 degrees
