500 men took dip in tank,which is 80m long and 50m broad. What is the rise iin the water level if the average displacement of water by a man is 4m^3

Let the rise in the level of water in the tank = h 

Now, 

The amount of water rises in the tank = Displacement of water taken by 500 persons.

⇒ 80 × 50  × h = 500 × 4 

⇒ 4000h = 2000 

Thus, the rise of level of water in the tank is 500mm.