A locksmith has n keys that open a total of n locks. Each key opens exactly one lock. It takes 15 seconds to determine whether a given key opens a given lock. If after a total of 3 minutes spent exclusively on determining whether or not the first k of the n keys fit a given lock, 40 percent of the keys have been tried then n=?
(A)12
(B)16
(C)18
(D)20
(E)30
submitted by vidya chichilli
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1 comment:
and is E
Determining one key takes 15 secs..therefore in 3 minutes(180 secs) 12 keys are determined..and 12 keys is 40% of total keys ..hence total keys are 30
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