Results 1 to 6 of 6
Like Tree4Thanks
  • 2 Post By SlipEternal
  • 2 Post By JeffM

Math Help - Odd one out in sequence

  1. #1
    Newbie
    Joined
    Dec 2013
    From
    Bangalore, India
    Posts
    9
    Thanks
    1

    Odd one out in sequence

    Hello!

    What number doesn't belong in the following sequence and why?
    7, 9, 13, 17, 24, 35

    Choose odd one out
    706 507 489 250
    It's 250 but don't know reason

    Choose odd one out
    232 431 612 813
    It's 813 but don't know reason

    Thanks in Advance.
    Last edited by lingping7; June 2nd 2014 at 11:11 PM.
    Follow Math Help Forum on Facebook and Google+

  2. #2
    MHF Contributor
    Joined
    Nov 2010
    Posts
    1,832
    Thanks
    703

    Re: Odd one out in sequence

    Since you have finite sequences, you can choose any one, and it works. For example, in the first sequence, I can say 24 doesn't belong since every other number listed is the product of at most two primes (but 24 is the product of four primes 24 = 2^3\cdot 3).

    Alternately, if I consider differences between adjacent terms, I find they are 9-7=2, 13-9=4, 17-13=4, 24-17=7, 35-24=11. Since 35 is the only one that is more than 10 greater than the previous term, I can say that is the one that doesn't belong.

    I can create an argument for any term in the sequence. There is no "correct" answer. There are only answer and reason pairs. For each number, I can find a reason why it doesn't belong.

    Here are some reasons for the numbers you picked for the next two finite sequences:
    1. Neither 250 nor 813 has an element proceeding it.
    2. 706 = 2\cdot 353, 507 = 3\cdot 13^2, 489 = 3\cdot 163, 250 = 2\cdot 5^3, so 250 is the only number among the four that is the product of more than three primes (not distinct)
    232 = 2^3\cdot 29, 431 = 431, 612 = 2^2\cdot 3^2\cdot 17, 813 = 3\cdot 271, so 813 is the only number among the four with two prime factors.
    3. 706-507 = 199, 507-489 = 16, 489-250 = 239, so 250 is the only one that is more than 200 less than the preceding number.
    431-232 = 199, 612-431 = 181, 813-612=201, so 813 is the only one that is more than 200 greater than the preceding number.

    The list of reasons why you might choose 250 and 813 respectively is endless.
    Thanks from JeffM and lingping7
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Senior Member
    Joined
    Feb 2014
    From
    United States
    Posts
    469
    Thanks
    192

    Re: Odd one out in sequence

    I do not understand why students are even given these problems. As SlipEternal says, there are an infinite number of correct answers.

    Here is an alternative for sequence 1:

    9 -7 = 2, 24 - 17 = 7, 35 - 24 = 11. If 13 is really supposed to be 12, then 12 - 9 = 3 and 17 - 12 = 5, and the numbers in the sequence differ by the sequence of primes.
    Thanks from Plato and lingping7
    Follow Math Help Forum on Facebook and Google+

  4. #4
    MHF Contributor

    Joined
    Aug 2006
    Posts
    18,605
    Thanks
    1574
    Awards
    1

    Re: Odd one out in sequence

    Quote Originally Posted by JeffM View Post
    I do not understand why students are even given these problems. As SlipEternal says, there are an infinite number of correct answers.
    There are busy work from a non-creative instructor.
    Follow Math Help Forum on Facebook and Google+

  5. #5
    Newbie
    Joined
    Dec 2013
    From
    Bangalore, India
    Posts
    9
    Thanks
    1

    Re: Odd one out in sequence

    Quote Originally Posted by SlipEternal View Post
    Since you have finite sequences, you can choose any one, and it works. For example, in the first sequence, I can say 24 doesn't belong since every other number listed is the product of at most two primes (but 24 is the product of four primes 24 = 2^3\cdot 3).

    Alternately, if I consider differences between adjacent terms, I find they are 9-7=2, 13-9=4, 17-13=4, 24-17=7, 35-24=11. Since 35 is the only one that is more than 10 greater than the previous term, I can say that is the one that doesn't belong.

    I can create an argument for any term in the sequence. There is no "correct" answer. There are only answer and reason pairs. For each number, I can find a reason why it doesn't belong.

    Here are some reasons for the numbers you picked for the next two finite sequences:
    1. Neither 250 nor 813 has an element proceeding it.
    2. 706 = 2\cdot 353, 507 = 3\cdot 13^2, 489 = 3\cdot 163, 250 = 2\cdot 5^3, so 250 is the only number among the four that is the product of more than three primes (not distinct)
    232 = 2^3\cdot 29, 431 = 431, 612 = 2^2\cdot 3^2\cdot 17, 813 = 3\cdot 271, so 813 is the only number among the four with two prime factors.
    3. 706-507 = 199, 507-489 = 16, 489-250 = 239, so 250 is the only one that is more than 200 less than the preceding number.
    431-232 = 199, 612-431 = 181, 813-612=201, so 813 is the only one that is more than 200 greater than the preceding number.

    The list of reasons why you might choose 250 and 813 respectively is endless.
    You are absolutely correct by saying that the reasons to choose a number is endless in a finite series. Thank you for getting that into my head!

    Though I just now said that there is no CORRECT answer, JeffM's answer for the 1st probably seems more appropriate.

    Anyway thank you guys
    Follow Math Help Forum on Facebook and Google+

  6. #6
    Super Member

    Joined
    May 2006
    From
    Lexington, MA (USA)
    Posts
    11,686
    Thanks
    617

    Re: Odd one out in sequence

    Hello, lingping7!

    These are more like riddles or trick questions.


    What number doesn't belong in the following sequence and why?
    . . 7, 9, 13, 17, 24, 35

    24 is the only even number.




    Choose odd one out.
    . . 706 507 489 250
    (It's 250, but don't know reason.)

    250 is the only number whose sum-of-digits is a one-digit number.




    Choose odd one out .
    . . 232 431 612 813
    (It's 813, but don't know reason.)

    813 is the only number whose sum-of-digits is a two-digit number.
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Replies: 2
    Last Post: August 24th 2010, 02:10 AM
  2. Replies: 0
    Last Post: July 4th 2010, 12:05 PM
  3. Replies: 2
    Last Post: March 1st 2010, 11:57 AM
  4. sequence membership and sequence builder operators
    Posted in the Discrete Math Forum
    Replies: 0
    Last Post: June 4th 2009, 03:16 AM
  5. Replies: 12
    Last Post: November 15th 2006, 12:51 PM

Search Tags


/mathhelpforum @mathhelpforum