Question 59646
suppose 's'  denotes arc length and 'r' denotes the radius of the circle,
then the ratio of any arc to the radius, s/r will be the radian measure of the central angle (theta) which that arc subtends.

So,  radian measure of central angle=s/r

here  s=20 inches
      r=4 inches

so radian measure of central angle=20/4= 5 inches