Since angle which is subtended by an arc at the centre of the circle is double the angle subtended by that arc at any point on the remaining part of the circle.

So, the reflex POR = 2 × PQR

We know the values of angle PQR as 100°

POR = 2×100° = 200°

∴ POR = 360°-200° = 160°

Now, in ΔOPR,

OP and OR are the radii of the circle

OP = OR

OPR = ORP

Now, we know sum of the angles in a triangle is equal to 180 degrees

POR + OPR + ORP = 180°

OPR + OPR = 180° - 160°

As OPR = ORP

2OPR = 20°

Thus, OPR = 10°