4. In one fortnight of a given month, there was a rainfall of 10 cm in a river valley. If the area of the valley is 7280 km2, show that the total rainfall was approximately equivalent to the addition to the normal water of three rivers each 1072 km long, 75 m wide and 3 m deep.

Firstly we will calculate the volume of rainfall :

Volume of rainfall   :

$=\ 7280\times \frac{10}{100\times 1000}$

$=\ 0.7280\ Km ^3$

And the volume of the three rivers is :

$=\ 3\left ( 1072\times \frac{75}{1000}\times \frac{3}{1000} \right )\ Km^3$

$=\ 0.7236\ Km ^3$

It can be seen that both volumes are approximately equal to each other.

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