I know that #180°# is #pi# radians (consider the length of a half circle arc; it spans an angle of #180°# and its length is #pir# or half the entire circle perimeter. But radians is (arc length)/(radius), so for your half circle you have that also spans #pir/r=pi# radians).
Now you can say that: #(180°)/(25°)# must have the same result as #pi/x# where #x# is the corresponding measure of #25°# in radians:
Finally: #x=(25°)/(180°)pi=5/36pi#