Well, in degrees, the arc is congruent to its central angle. If the radius is given, however, just find the circumference of the circle (C=πd). Then, take the measure of the central angle, and...
360 / 78.2 = 4.60 - Gives the number of "slices of pie" of this size in the entire circle
4.60 * 12 = 55.245 - Gives circumference of the circle
radius = circumference / 2pi
radius =...