H (depth) = 3 km = 3000 m.
Now, Volume of water that flows out of the circular pipe in 1 hour = πR2H
Let r m be the radius and h m be the depth (height) of a circular cistern, then
Now, Volume of water that flows in the circular cistern
= π r2h
= π x 5 x 5 x 2 = 50 π m3.
Hence,
Time taken to fill up the cistern