Thinking Challenge #13 – “Hotel Doors”
Here is this week’s thinking challenge. This puzzle is from the book BrainMatics Logic Puzzles by Ivan Moscovich. Enjoy!

Ten hotel doors are numbered from 1 to 10; all are closed. The cleaning lady walks by and opens every door. Then a second cleaning lady walks by and closes all the even-numbered doors. Then the repairman walks by and changes the state of every door whose number is a multiple of three. Then another person changes the state of every door whose number is a multiple of four, and so on, until ten people have walked by.
Which doors will be open after the tenth person has walked by? Can you also guess as to which doors would be open at the end if there were 100 doors involved in the same procedure?
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