You have 130 keys and only 1 unlocks a certain door. What are the odds you draw that key out of a the pile of 130 on your nth try?
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Originally Posted by mathman88 You have 130 keys and only 1 unlocks a certain door. What are the odds you draw that key out of a the pile of 130 on your nth try? Draw a tree diagram. You will quickly see that Pr(N = n) = 1/130.
Last edited by mr fantastic; Feb 17th 2009 at 11:10 PM. Reason: Replaced X with N
I wrote the question down wrong... here it is corrected: You have 130 keys and only 1 unlocks a certain door. What are the odds you draw that key out of a the pile of 130 BY your nth try?
Originally Posted by mathman88 I wrote the question down wrong... here it is corrected: You have 130 keys and only 1 unlocks a certain door. What are the odds you draw that key out of a the pile of 130 BY your nth try? My previous reply is relevant: $\displaystyle \Pr(N \leq n) = \Pr(N = 1) + \Pr(N = 2) + .... + \Pr(N = n) = \cdot $
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