There are 100 doors in a row, all doors are initially closed. A person walks through all doors multiple times and toggles (if open then close, if close then open) them in following way: In first walk, the person toggles every door In second walk, the person toggles every second door, i.e., 2nd, 4th, 6th, 8th. In the third walk, the person toggles every third door, i.e. 3rd, 6th, 9th; In 100th walk, the person toggles 100th door. Which doors are open in the end?