JPCosta » mensa http://www.joaocosta.org/blog Mais um Português na Polónia! Fri, 01 Jul 2011 20:53:08 +0000 en hourly 1 http://wordpress.org/?v=3.3 Brain teaser: 3000 doors http://www.joaocosta.org/blog/2008/06/brain-teaser-3000-doors/ http://www.joaocosta.org/blog/2008/06/brain-teaser-3000-doors/#comments Sun, 01 Jun 2008 11:47:57 +0000 jpcosta http://www.joaocosta.org/blog/?p=26 I found this brain teaser on the portuguese mensa forum and I think it is really interesting:

There are 3000 closed door’s and 3000 doorman’s. The first doorman opens all doors. The second doorman closes every second door. The third doorman goes to every third door and opens it if it was previously closed and closes it if it was previously opened etc..

How many doors remain closed after the 3000th doorman ?

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