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by knight247 » Tue Oct 04, 2011 1:10 am
A password to a certain database consists of digits that cannot be repeated.If the password is known to consist of at least 8 digits and it takes 12 seconds to try one combination , what is the amount of time , in minutes , necessary to guarantee access to database?
A. 8!/5
B. 8!/2
C. 8!
D. 10!/2
E. 5/2.10!

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by sl750 » Tue Oct 04, 2011 1:30 am
ASsuming that a password can start with a 0, it will take


12 seconds for 10*9*8*7*6*5*4*3 combination
For 1 minute it is 5/2*10!
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by knight247 » Tue Oct 04, 2011 4:37 am
U haven't considered the scenario where the password could have 9 or 10 digits.
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by GmatKiss » Tue Oct 04, 2011 5:04 am
IMO:D

Total # of passwords possible for 8 digits is 10!/2
Total # of passwords possible for 9 digits is 10!
Total # of passwords possible for 10 digits is 10!

Time needed to guarantee access to database is (10!/2+10!+10!)*1/5= 10!/2 minutes.

Answer: D.

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