## 100 locker riddle

 Tweet Puzzle: There are 100 closed lockers in a hallway. A man begins by opening all the 100 lockers. Next, he closes every second locker. Then he goes to every third locker and closes it if it is open or opens it if it is closed (eg, he toggles every third locker). After his 100th pass in the hallway, in which he toggles only locker number 100, how many lockers are open? Solution: There are 10 lockers open, and here’s why: Door n will be toggled (1 toggle = door opened or door closed) x times, where x is the number of factors of n. That is, door 20 will be toggled on round 1, 2, 4, 5, 10, and 20. Question: When would a door be left open? Answer: A door is left open if x is odd. You can think about this by pairing factors off as an open and a close. If there’s one remaining, the door will be open. Question: When would x be odd? Answer: x is odd if n is a perfect square. Here’s why: pair n’s factors by their complements. For example, if n is 36, the factors are (1, 36), (2, 18), (3, 12), (4, 9), (6, 6). Note that (6, 6) only contributes 1 factor, thus giving n an odd numer of factors. Question: How many perfect squares are there? Answer: There are 10 perfect squares. You could count them (1, 4, 9, 16, 36, 49, 64, 81, 100), or you could simply realize that you can take the numbers 1 through 10 and square them (1*1, 2*2, 3*3, ..., 10*10). CLICK HERE FOR SOLUTION POST YOUR OPINION IF YOU HAVE BETTER SOLUTION
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Puzzle:

There are 100 closed lockers in a hallway. A man begins by opening all the 100
lockers. Next, he closes every second locker. Then he goes to every third locker
and closes it if it is open or opens it if it is closed (eg, he toggles every third
locker). After his 100th pass in the hallway, in which he toggles only locker
number 100, how many lockers are open?

Solution:

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