To find the length of the minor arc of a circle with a given circumference and a central angle, we can use the following formula for arc length:
\[ \text{Arc length} = \frac{\theta}{360} \times C \]
where:
- \( \theta \) is the central angle in degrees,
- \( C \) is the circumference of the circle.
Given that the circumference \( C = 20\pi \) and the central angle \( \theta = 90 \) degrees, we can substitute these values into the formula.
\[ \text{Arc length} = \frac{90}{360} \times 20\pi \]
Simplifying this, we find:
\[ \text{Arc length} = \frac{1}{4} \times 20\pi = 5\pi \]
Thus, the length of the minor arc is \(\boxed{5\pi}\).
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