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# The safe lock consists of three disks: the first one marked

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The safe lock consists of three disks: the first one marked [#permalink]

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19 Dec 2003, 02:24
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The safe lock consists of three disks: the first one marked from 0 to 9, the second marked with even digits, and the third marked with 1 and 0. A certain guy has a one attempt to open the safe by rotating disks randomly. Assuming that there is the only correct combination to open the safe, what is the probability that the guy will not open the safe?
Director
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19 Dec 2003, 05:32
anvar, agree with the method offered, but think that on second ring we should coun also 0 which is even, as far as i know, which makes the possible combinations 100( 10x5x2), not 80...
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19 Dec 2003, 05:34
BG wrote:
anvar, agree with the method offered, but think that on second ring we should coun also 0 which is even, as far as i know, which makes the possible combinations 100( 10x5x2), not 80...

Thanks BG, good to know that "zero" is even
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19 Dec 2003, 10:03

(analogy with 750+ on GMAT)
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19 Dec 2003, 10:06
99% is OK
to be perfect, let's transform the number into 0.99.
according to math definition of probability, 0<=P<=1
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05 Jan 2004, 15:50
Probability that he will open the lock
= 1/10 * 1/5 * 1/2

probability that he wont open the lock = 1 - 1/100 = 0.99
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