There are 100 windows in a room. 100 persons are asked to enter a room one by one and open or close the windows according to a method. The first person has to close all windows. 2nd person has to change the state (close if open and open if close) all even numbered windows. 3rd person has to change the state of the windows that are multiple of 3 and so on. How many windows will remain open after 100th person's turn ?