Vol of water displaced by one person = 0.o4m^3
Vol of water displaced by 500 persons = 500 *4/100 m^3
= 20 m^3
Length of tank = 80m
Breadth of tank = 50 m
Height of water displaced = h
Now, Vol of water displaced = 80*50*h
A.T.Q
80 * 50 *h = 20
h = 1/200 m = 0.5cm