4 Answers

pi/3
You can use 13 to determine the quadrant the angle is in.

13 pi / 3 is  4 1/3 pi, two full times around plus pi/3 but in a clockwise direction so the reference angle would be pi/3

✰Answer✰
60 degrees
✐Derivation✐
Know that:
Reference angle  angle formed by the coterminal ray and xaxis.
So
13π/3 = 13(180)/3 = 780
1080  780 = 300
360  300 = 60

Whatever floats your boat.
Your Welcome,
MuiscMan
ps i like cherry pi