A farmer runs a pipe of internal diameter 20 cm from the canal into a cylindrical tank in his field which is 10 m in diameter and 2 m deep. If water flows through the pipe at the rate of 3 km / h, in how much time will the tank be filled?
Let the time-taken by the pipe to fill the tank be x hrs.
Since the water is flowing at a rate of 3 kmph, therefore
Length of water column in x hrs is
3 × x = 3x km = 3000x m
Therefore the length of the pipe is 
Therefore, the volume of water flowing through the pipe in x hours, V’ = π r2 h
= π (0.1)2 (3000x) m3
Also, volume of water that falls into the tank in x hours = π r2 h
= π (5)2 (2)
The volume of water flowing through the pipe in x hours = volume of water that falls into the tank in x hours
∴ π (0.1)2 (3000x) = π (5)2 (2)
⇒ 30x = 50
= 1hr 40min