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Algebra (probably easy) problem

  1. Jan 20, 2009 #1
    1. The problem statement, all variables and given/known data

    There are two equations and 3 variables and I'm asked to solve for the numerical value of a third expression:

    We are told that

    P/R=3/5

    P/T=9/10

    The question asks for the value of (R+T)/R

    2. Relevant equations

    The correct answer is 5/3 but I have no idea how to get there. This is from a GRE practice test.



    3. The attempt at a solution

    I tried addin' em together. Also solving for "P" to get rid of it doesn't work right away.
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Jan 20, 2009 #2

    Mark44

    Staff: Mentor

    Try solving the first equation for R and the second for T. Then substitute those values into the expression you need to evaluate.
     
  4. Jan 20, 2009 #3
    oh I did not see in advance that the P's would cancel out like that and I'd get a number. I never would've thought to do that. Could you explain how you saw that?
     
  5. Jan 20, 2009 #4
    When you have equations that share variables, you can always do that setup to eliminate the common variable.
     
  6. Jan 20, 2009 #5
    Yes but you couldn't give a numerical value for just R+T, in this case you'd be left with 25P/9
     
  7. Jan 20, 2009 #6

    Mark44

    Staff: Mentor

    This is to be expected. You can't get a numerical value for R unless you know P, and you can't get a numerical value for T unless you know P. Since you don't know P, you are not going to be able to get a numerical value for (R + T)/R.
     
  8. Jan 20, 2009 #7
    You have 2 equations and 3 unknowns. Chances are, you will have a free variable.
     
  9. Jan 20, 2009 #8
    Actually I believe this is a special case where you can get a numerical answer just because the P's happen to cancel.
     
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