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GRE Permutations: Practice Question of the Week #37 Answer

Here’s the answer to yesterday’s practice question, thanks for sending in all of your answers!


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11 Responses to GRE Permutations: Practice Question of the Week #37 Answer

  1. spondon June 19, 2012 at 3:39 am #

    Hi Chris,
    If the question ask that password will be at least 9 digits number then what will be the answer? 18*9!

    • Chris Lele
      Chris June 19, 2012 at 3:28 pm #

      Hi Spondon,

      Do you mean minus the repeating constraint, i.e., a number can repeat? If so, then the answer would actually be 9^9 x 10^9. Again, I’m not quite sure what you are asking, but I hope that helps :).

      • spondon June 20, 2012 at 2:45 am #

        no repeating constraint. just for the significant 9 or 10 digits “number” password. because in some arrangement the starting number will be “0”. I am asking for this case.



        • Chris Lele
          Chris June 20, 2012 at 3:27 pm #

          Hi Spondon,

          Hmmm…I’m still not quite sure what you are asking. Could you take another stab at rephrasing your question :).

  2. Jim February 23, 2012 at 5:42 pm #

    Hi, I have a question regarding why it is 2 x 10! Since the question ask for permutation of at least 9 digits, which is 9x8x… and so on. Why is it necessary to do this twice? If it’s also 10X9x and so, then it could also be 11x10x9, and so on. In theory, it could also be 3x 10! or 4x 10! So where I fail to understand is how do we know from the question that it should be 2 x 10! and not 3 x 10! etc ? Thanks.

    • Chris Lele
      Chris February 24, 2012 at 12:06 pm #

      Hi Jim,

      The twist to this problem is the number zero. The password requires a digit and there are 10 digits, including zero: 0, 1, 2, 3…9.

      The question also states that the password has to be at least 9-digits long. So we have two possible passwords: a 9-digit password and a 10-digit password. Each has 10! different passwords so we get 2 x 10!.

      Hope that helps 🙂

      • Jim February 24, 2012 at 1:42 pm #

        Thanks Chris! I just signed up for the Magoosh premium service. Will be taking my GRE for the first time hopefully in the upcoming first weeks of April. To say I have an anxiety for taking the GRE would be an understatement. I’ve been putting it off for over 2 years now, I would’ve had my masters by now, lol. I’ve taken College GRE prep course when the old GRE was was in circulation. I have to say that, that prep course wouldn’t have helped me much particularly in the Quant section, it was 6 weeks long and barely scratching the surface! It didn’t help with most of the complex problems.

        So having signed up for Magoosh, hopefully it will prepare me in 30-35 days. I would like to devote 15 hours a week for 4 to 5 weeks and take my first attempt. Hope to at least get in the above 150 range on each section, that’s my target, not too unrealistic, but not too pessimistic either. Because I know that if I plan it out for 2 to 3 months, I would never be focused for the long haul and miss the deadline for fall admission, once again!

        • Chris Lele
          Chris February 24, 2012 at 3:44 pm #

          Hi Jim,

          Sounds like a good plan! I agree – it’s a lot easier to stick to an agressive 30-day plan than it is a 2-3 month plan.

          I think you will be happy with Magoosh’s quant – we help you become confident in all the fundamentals in the lesson videos. Then you apply that knowledge to question that go from the easy to the mind-blowing-ly complex.

          I think your 15 hr. plan should def. get you to 150+ in each section. You may want to take the PowerPrep II test in the next couple of weeks to see where you are scoring.

          Let me know if I can answer any other questions :).

      • Kalyani July 10, 2012 at 12:01 pm #

        As I understand the password here is supposed to be atleast 9 digit long.
        So isn’t 012345678 considered as an 8 digit number? If yes, then I think the possible 9 digit passwords would be 9 x 9! and possible 10 digit ones would be 9 x 9!. Do correct me if I am wrong. Thanks!

        • Chris Lele
          Chris July 11, 2012 at 6:12 pm #

          Hi Kalyani,

          012345678 has nine digits including the zero. Of course in number land that would not be a valid number (no integer, besides ‘0’, starts with a zero). In password land you can start with a zero.

          Hope that helps!

  3. Rubaet February 8, 2012 at 8:13 pm #

    Yes, Order matters in this case.

    Because 123456789 and 987654321 are two different passwords.

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