Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Mean and standard deviation for linear combinations

  1. Jan 22, 2009 #1
    1. The problem statement, all variables and given/known data

    Data set in X with mean xbar = 100 and standard deviation Sx = 10

    Find ybar and Sy for 2(Yi-5)/10 + 7



    2. Relevant equations



    3. The attempt at a solution

    All the problems I have seen are in the form yi = axi + b in which case the mean ybar = a(xbar) + b and Sy = aSx

    What does Yi represent exactly? How does it relate to xi?
     
  2. jcsd
  3. Jan 23, 2009 #2

    Mark44

    Staff: Mentor

    I don't see how you can find ybar and Sy unless you know the relationship between X and Y.

    You show an expression 2(Yi - 5)/10 + 7 = (Yi - 5)/5 + 7. Could this be Xi?

    If that's the case, then you can solve for Yi.

    Xi = (Yi - 5)/5 + 7
    ==> Xi - 7 = (Yi - 5)/5
    ==> 5(Xi - 7) = (Yi - 5)
    ==> 5(Xi - 7) + 5 = Yi
    or
    Yi = 5Xi - 30
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook