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The volume of one person = 0.04 m³ , be cause when the person takes a dip in the tank, the amount of water displaced is equal to the volume of the person.

500 people take dip at the same time, then water displaced = 500 * 0.04 = 20 m³

the water rises inside the tank of the area of 80 m * 50 m = 4000 m².

The height to which water rises = volume / area = 20/ 4000 = 1/200 m =0.005 m
      = 5 millimeters

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i can't able to understand.
simple multiplication of quantities. that is all. simple arithmetic. volume in a rectangular tank = base area of tank * height of water in it.
On top of your house or appartment building, you have a tank, right? so if some object of volume 0.04 m^3 is put inside it, then water level will rise by that much.. then you get the height of increase by finding 0.04 / area of tank bottom.