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(Diagram not to scale.)

In the triangle $PQR$, the angle $QPR=40^{\circ}$ and the internal bisectors of the angles at $Q$ and $R$ meet at $S$, as shown. What is the size of angle $QSR$?

If you liked this problem, here is an NRICH task which challenges you to use similar mathematical ideas.

*This problem is taken from the UKMT Mathematical Challenges.*