Question

A river 3m deep and 40m wide is flowing as the rate of 2km per hour. how much water will fall into the sea in a minute?

Answer 1

Given, a river 3m deep and 40m wide is flowing at the rate of 2km per hour.

Area of cross section of the river = 3 * 40 = 120 m2

Now, volume of water flowing through this cross section in every minute = Area * rate

                                                                                                 = 120 * 2 km/hr

                                                                                                 = 120 * (2000/60) m/min

                                                                                                 = 2 * 2000

                                                                                                 = 4000 m3

Since 1 m3 = 1000 litres

So, 4000 m3 = 4000 * 1000 litres = 4000000 litres

So, in 1 minute, water will fall into the sea is 4000000 litres

Answer 2

imagining the river to be a cuboid, the volume of water in flowing into sea in a minute is equal to the volume of a cuboid with height = 3m, breadth = 40m and length = speed * time.
So, 
 water falling into the sea = lbh
                                         =3*40*(2000m/h)*(2/60)h
                                         =8000metercube