100 degrees, because since it is given that PR and QS are diameters, their arcs are 180 degrees each. To find SR, first you must find arc PS. to find PS, use the 40 degrees of the inscribed angle to find the arc of PS, which is double the amount (80 degrees). Then the other half of the arc must be 100 degrees since it has to be 180 degrees in total. And so 100 degrees is your answer.
