Explanation:
the length of the full circle arc of 360° (also called circumference) is
2pi×r
r being the radius.
so, if we have only a fraction of the full circumference defined by the same fraction of 360° via the given angle, we then know that the length of that arc is
2pi×r × angle/360
in our case we have
2pi × 20 × 45/360 = 2pi × 20 × 1/8 = pi×20×1/4 = 5pi
the length of the arc is 5pi inches.