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ID number.

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Joined: 18 Feb 2010
Posts: 20
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Kudos [?]: 8 [0], given: 2

Re: ID number. [#permalink]  04 Mar 2010, 22:24
Step 1: Total number of ways = 10*10*10*10*10 = 10^5
Step 2: First choose three places for three 2s which can be done in 5C3 ways. (No point arranging these 2s amongst themselves) = 10 ways
Step 3: Now for the remaining two positions there are 9 numbers (as 2 is already used) each to be placed which can be done in 9*9 ways.
Step 4: favorable ways = 10*9*9 = 810

Thus probability = favorable ways / total ways = 810/10^5=0.0081 or 0.81%

Hope the solution helps
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Re: ID number.   [#permalink] 04 Mar 2010, 22:24
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