P,Q,R P, Q, R\,P,Q,R and S S\,S are points on the circumference of a circle.
Angle QRS=115∘QRS = 115^\circQRS=115∘ Angle RSP=78∘RSP = 78^\circRSP=78∘
Find the size of angle PQRPQRPQR.
Give a reason for your answer.