Suppose you're in a hallway lined with 100 closed lockers.
You begin by opening every locker. Then you close every second 
locker. Then you go to every third locker and open it (if it's 
closed) or close it (If it's open).  (Let's call this action 
toggling a locker.) Continue toggling every nth locker on pass 
number n.  After 100 passes, where you toggle only locker #100,
How many lockers are open?

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