Step 1: Understanding volume flow rate concept.
The volume of water flowing per second is given by:
\[
Q = A \cdot v
\]
where \(A\) is cross-sectional area of pipe and \(v\) is velocity of flow. This gives volumetric discharge in cm$^3$/s.
Step 2: Calculate cross-sectional area of pipe.
Diameter = 20 cm, so radius:
\[
r = 10 \, \text{cm}
\]
Area:
\[
A = \pi r^2 = \pi \times 10^2 = 100\pi \, \text{cm}^2
\]
Step 3: Multiply area with velocity.
Given velocity:
\[
v = \frac{50}{\pi} \, \text{cm/s}
\]
So flow rate:
\[
Q = 100\pi \times \frac{50}{\pi} = 5000 \, \text{cm}^3/\text{s}
\]
Step 4: Convert into volume per minute.
\[
Q_{1\,min} = 5000 \times 60 = 300000 \, \text{cm}^3
\]
Step 5: Convert cm$^3$ to litres.
\[
1000 \, \text{cm}^3 = 1 \, \text{litre}
\]
\[
300000 \, \text{cm}^3 = 300 \, \text{litres}
\]
Step 6: Final conclusion.
\[
\boxed{300 \, \text{litres}}
\]