What is the measure of the central angle of a circle with radius 30 centimeters that intercepts an 18π centimeters arc?

the circumference of the circle when r=30 is C=60π
the arc is 18π, it is 18π/60π=3/10 of the whole circle
the arc is 3/10 of 360 degree, which is 108 degree
the central angle measures the same as the arc, 108 degree.