Question

water is being pumped out through a circular pipe whose internal diameter is 7 cm. if the flow of water is 72 per second how many litres of water are being pumped out in 1 hour

Answer 1

According to question

Diameter of pipe  = 7 Cm
Thus

Radius of pipe  = r = 7/ 2 Cm
Let length of pipe be L

Rate of flow of water through pipe = 72 Cm per sec

Thus
in 1 sec  water flowed  = 72 Cm
Thus 
in 60 sec    = 72 * 60
in 1 hr  = 72 * 60 * 60

So clearly 
Volume of water flowed = pi r^2 H
                                      = 22/7 * 49/4 * 72 * 60 * 60
                                      = 9979.2 Liter

Answer 2

Diameter of circular pipe = 7 cm
so, radius of circular pipe (r) = 7/2 cm

Length of water per sec = water flow through pipe per sec( h ) = 72cm

now, volume of water per second = volume of cylinder per second
= π r²h = 22/7 × 7/2 × 7/2 × 72
= 77 × 36 = 2772 cm³

we know,
1 litre = 1000cm³
So, volume of water per sec = 2.772 litres

Now, volume of water in one hour = volume of water per second × 1hour
= 2.772 × 3600 litres [ 1 hour= 3600 sec ]
= 9979.2 litres