Results 1 to 6 of 6

Math Help - ratio

  1. #1
    MHF Contributor
    Joined
    Sep 2008
    From
    West Malaysia
    Posts
    1,261

    ratio

    If P(n-1,3):P(n,3)=1:9 , find n ..
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Lord of certain Rings
    Isomorphism's Avatar
    Joined
    Dec 2007
    From
    IISc, Bangalore
    Posts
    1,465
    Thanks
    6
    Quote Originally Posted by mathaddict View Post
    If P(n-1,3):P(n,3)=1:9 , find n ..
    Have you tried applying the definition of P(n,k)?

    P(n,3) = \frac{n!}{(n-3)!}
    P(n-1,3) = \frac{(n-1)!}{(n-4)!}

    and thus we have P(n-1,3):P(n,3) = n:n-3....

    Can you continue ?
    Follow Math Help Forum on Facebook and Google+

  3. #3
    MHF Contributor
    Joined
    Sep 2008
    From
    West Malaysia
    Posts
    1,261

    Re :

    My attempt :

    After simplifying , i got

    \frac{(n-1)(n-2)(n-3)}{n(n-1)(n-2)}=\frac{1}{9}

    \frac{n-3}{n}=\frac{1}{9}

    9n-27=n

    n=27/8 ---- my answer is wrong .. Can someone pls spot my mistake . THanks .
    Follow Math Help Forum on Facebook and Google+

  4. #4
    Flow Master
    mr fantastic's Avatar
    Joined
    Dec 2007
    From
    Zeitgeist
    Posts
    16,948
    Thanks
    5
    Quote Originally Posted by mathaddict View Post
    My attempt :

    After simplifying , i got

    \frac{(n-1)(n-2)(n-3)}{n(n-1)(n-2)}=\frac{1}{9}

    \frac{n-3}{n}=\frac{1}{9}

    9n-27=n

    n=27/8 ---- my answer is wrong .. Can someone pls spot my mistake . THanks .
    I'm sure n is meant to be a positive integer. It looks to me like the question has a typo in it. What answer does the book give?
    Follow Math Help Forum on Facebook and Google+

  5. #5
    MHF Contributor
    Joined
    Sep 2008
    From
    West Malaysia
    Posts
    1,261

    Re :

    Well , let me retype the question .

    P(n-1,3):P(n,3)=1:9 , find n .

    The choices are

    (a) 6
    (b) 7
    (c) 8
    (d) 9
    (e) 4
    Follow Math Help Forum on Facebook and Google+

  6. #6
    Flow Master
    mr fantastic's Avatar
    Joined
    Dec 2007
    From
    Zeitgeist
    Posts
    16,948
    Thanks
    5
    Quote Originally Posted by mathaddict View Post
    Well , let me retype the question .

    P(n-1,3):P(n,3)=1:9 , find n .

    The choices are

    (a) 6
    (b) 7
    (c) 8
    (d) 9
    (e) 4
    I meant a typo in the book (or wherever the question came from), not a typo in how you typed it. Test each option - do any of them work ....?
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. [SOLVED] Ratio
    Posted in the Algebra Forum
    Replies: 8
    Last Post: June 19th 2011, 03:59 AM
  2. Ratio
    Posted in the Algebra Forum
    Replies: 1
    Last Post: May 2nd 2010, 03:05 AM
  3. ratio once again
    Posted in the Trigonometry Forum
    Replies: 2
    Last Post: April 19th 2010, 10:43 AM
  4. Ratio for curve and point-ratio form
    Posted in the Algebra Forum
    Replies: 4
    Last Post: January 11th 2010, 12:07 AM
  5. Finding n given the ratio of nCr to another ratio
    Posted in the Discrete Math Forum
    Replies: 1
    Last Post: October 5th 2009, 07:03 PM

Search Tags


/mathhelpforum @mathhelpforum