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