Question

A river 2meters deep and 45 metres wide is flowing at the rate of 3km per hour. Find the amount of water that runs into the sea per munute

Answer 1

hey there, i could have solved it in 2-3 steps but since you asked for proper method, i am solving it in detail. hope that's ok? 
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before starting, lets recall in earlier classes that i m^3 = 1 cubic liter [in vol q's] 

in this case, you need to find the volume of flowing water that's going into the sea. 

for a moment, suppose it would have been a huge tank instead of a river. could you have found the volume of the same? 

of course you would have. its l*b*h. 

herein, 

depth {height} = 2 m 

width = 45 m 

so, you know two o/o three dimensions. 

how much is the length is unknown to you. 

actually it isn't unknown. it is given as 3000 meters 

[ here the length has been given to be = 3 km =3 000 meter. [water flowing through this distance entered at a place 3 km before & flowed through this giant tank of 45*2*3000 cu meters capacity; in 1 hour]] 

so how much water went into the sea? 

the same amount which come into this giant tank 3 km back @ 3 km /hour. 


so, we'll find amt of water in 1 hour & then convert it in per minutes. 

in1 hour, amt of water = 2*45*3000 cu liter [m^3 = l^3 conversion] 

= 270000 cu liter 

in 1 minuter, amt of water into the sea = 270,000 / 60 

= 4500 cubic liter. 


thus, in 1 minute, the amount of water into the sea = 4500 cubic liter

Answer 2

h=2m ,b=45m
l=3km =3×1000 m=3000m
amount of water flow in 1 hour =2×45×3000
=270000
60 min =1 hr
1 min =1/60
amount of water flow in 1 min =270000/60
= 4500 litre³