Results 1 to 4 of 4

Math Help - Probably very easy but i'm stupid

  1. #1
    Newbie
    Joined
    Mar 2008
    Posts
    6

    Probably very easy but i'm stupid

    The average of three numbers is 57. The average of two of them is 45. What is the third number.

    I tried defining each number as a random variable x, y, z and doing

    (x+y+z)/3 = 57 and (x+y)/2 = 45 and solving simulataneously but couldn't get it to work.

    any ideas?
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Forum Admin topsquark's Avatar
    Joined
    Jan 2006
    From
    Wellsville, NY
    Posts
    9,853
    Thanks
    321
    Awards
    1
    Quote Originally Posted by IndoorKid View Post
    The average of three numbers is 57. The average of two of them is 45. What is the third number.

    I tried defining each number as a random variable x, y, z and doing

    (x+y+z)/3 = 57 and (x+y)/2 = 45 and solving simulataneously but couldn't get it to work.

    any ideas?
    For starters one problem is that there is not a unique answer:
    x + y + z = 171
    and
    x + y = 90

    Solving the second equation for y gives
    y = 90 - x

    Inserting this into the first equation gives:
    x + (90 - x) + z = 171

    90 + z = 171

    z = 81

    So there are a range of possible values for y, subject to the restiction that y cannot be negative, and only one score we can predict with certainty: the 81.

    -Dan
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Member
    Joined
    Nov 2007
    Posts
    100
    We assume:

    Average of two numbers is 45, so the total of numbers is 45 * 2 = 90
    Average of 3 numbers is 57 so the total is 57 * 3 = 171

    x + y + z = 171 (1)
    x + y = 90 (2)

    Subtract (2) from (1):

    x + y + z - x - y = 171 - 90
    Then, z = 81


    Ooops.. didn't know there already was a reply.
    Follow Math Help Forum on Facebook and Google+

  4. #4
    Newbie
    Joined
    Mar 2008
    Posts
    6
    thanks heaps guys
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. am i being stupid?
    Posted in the Algebra Forum
    Replies: 2
    Last Post: June 26th 2008, 04:46 AM
  2. 2 Stupid Questions? Can anyone do these?
    Posted in the Statistics Forum
    Replies: 2
    Last Post: March 8th 2008, 09:54 AM
  3. Stupid, Easy Anti-Derivative Question.
    Posted in the Calculus Forum
    Replies: 12
    Last Post: August 29th 2007, 09:49 PM
  4. stupid me
    Posted in the Algebra Forum
    Replies: 9
    Last Post: September 5th 2006, 06:42 PM

Search Tags


/mathhelpforum @mathhelpforum