Water in a rectangular reservoir having base $80\ m$ by $60\ m$ is $6.5\ m$ deep. In what time can the water be emptied by a pipe of which the cross-section is a square of side $20\ cm$, if the water runs through the pipe at the rate of $15\ km/hr$.
Given:
Water in a rectangular reservoir having base $80\ m$ by $60\ m$ is $6.5\ m$ deep.
The water runs through the pipe at the rate of $15\ km/hr$.
To do:
We have to find the time in which the water can be emptied by a pipe of which the cross-section is a square of side $20\ cm$.
Solution:
Length of the reservoir $(l) = 80\ m$
Breadth of the reservoir $(b) = 60\ m$
Depth of the reservoir $(h) = 6.5\ m$
Therefore,
Volume of water in the reservoir $= lbh$
$= 80 \times 60 \times 6.5$
$= 31200\ m^3$
Area of cross-section of the mouth of the pipe $= 20 \times 20$
$= 400\ cm^2$
$=\frac{400}{10000}\ m^2$
$=\frac{4}{100} \mathrm{~m}^{2}$
Speed of the water $=15 \mathrm{~km} / \mathrm{h}$
Length of flow of water in the pipe $=\frac{\text { Volume }}{\text { Area of cross - section }}$
$=\frac{31200 \times 100}{4}$
$=780000$
$=780 \mathrm{~m}$
Time taken $=\frac{\text { Length }}{\text { Speed }}$
$=\frac{780}{15}$
$=52$ hours
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