Thousand Door Hallway

---

In a hallway there is a thousand doors that are all closed. On the first day, all the doors are changed (if a door is open, then it is closed, and vice versa). On the second day, every other door is changed. On the third day, every third door is changed, on the fourth, every fourth, et cetera. After 1000 days, how many doors are open?

---

Home

Answer