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Word problem confusion

  1. Jun 17, 2013 #1
    I will occasionally get problems wrong because the "right" answer should be y=nx, but I put x=ny, is the program being nitpicky or am I somehow misreading the problem?
     
  2. jcsd
  3. Jun 17, 2013 #2

    Redbelly98

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    Since those are two different equations (unless n happens to equal 1), there is nothing nitpicky about accepting only one of them as correct. It sounds like you may be misreading the problem, but without showing us a specific example it is difficult to say with certainty.
     
  4. Jun 17, 2013 #3

    Mark44

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    The program is not being nitpicky. You might be misreading the problem or there might be something else going on. Without more information, it's hard to say.
     
  5. Jun 17, 2013 #4
    I've run into problems where y comes before x, but have to resort to just going with what is stated as the right answer for those kinds of problems without really understanding why I put y first. Is there a phrase I can look for or something?
     
  6. Jun 17, 2013 #5

    Office_Shredder

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    Can you give a specific example? There's no phrase you should look for, you should just properly understand the problem.

    Is this an example where you would get it wrong? There are x people, and each of them are holding 2 apples. Let y be the number of apples. Which of the following is true?
    y=2x
    x=2y
     
  7. Jun 17, 2013 #6
    To my mind, if there are x people and y apples, x would come first because that is what was stated first so in my mind x= 2y
     
  8. Jun 17, 2013 #7

    Mark44

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    I think you might be getting lost with abstract symbols.

    If there are 8 people (x = 8) and each of them has 2 apples, then how many apples are there in all?
     
  9. Jun 17, 2013 #8
    Solving it as 8=2y I get 4, shouldn't it be 16? Or am I again getting lost? I think I may see where you're going
     
  10. Jun 17, 2013 #9

    Office_Shredder

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    OK if I had rewritten the equations as
    2y = x
    2x = y

    would you have picked the 2x=y choice then?
     
  11. Jun 17, 2013 #10

    Mark44

    Staff: Mentor

    The fact that x appears before y should not steer you toward writing x = ...
     
  12. Jun 17, 2013 #11

    Mark44

    Staff: Mentor

    Yes, it should be 16. If there are 8 people, and each has 2 apples, then the total number of apples couldn't possibly be 4, could it?

    Furthermore, only one formulation makes sense in terms of the units. Each variable (or number) also has units.


    x (persons)
    2 (apples/person)
    y (apples)

    If you write x = 2y, the units are saying persons = apples/person * apples. The left and right sides don't agree - the right side would be (apples)2/person, which is nonsensical.

    If you write y = 2x, the units are saying apples = apples/person * person, so the left and right sides represent the same unit - apples.
     
  13. Jun 17, 2013 #12
    Exactly. The correct answer is 16. Of the two equations Office_Shredder offered to you, the one you chose yielded an incorrect answer. Therefore, the equation you chose is not an accurate model of the situation described. Whenever possible, you should test your equation as an accurate model.

    You are placing too much importance on the order in which variables appear in the phrasing of the problem. Consider the following two (equivalent) phrasings:

    1. There are [itex]x[/itex] people and [itex]y[/itex] apples. Each person holds two apples.

    2. There are [itex]y[/itex] apples and [itex]x[/itex] people. Each person holds two apples.

    Each phrasing corresponds to an equation. The meaning of the phrasings are identical. Therefore, the corresponding equations should be identical.

    In modeling the situation described in a world problem with a mathematical equation, you may find it helpful to gradually transition from the English language to the language of mathematics. For example, we could express the relationship between the number of people and the number of apples as the following English sentence:

    The number of people is twice the number of apples.

    Now, begin replacing the English components with appropriate mathematical symbols. We will start by replacing the phrase "the number of people" with the variable [itex]y[/itex] and continue from there.

    [itex]y[/itex] is twice the number of apples.

    [itex]y[/itex] is twice [itex]x[/itex].

    [itex]y[/itex] is [itex]2x[/itex].

    [itex]y[/itex] = [itex]2x[/itex].
     
  14. Jun 17, 2013 #13
    I just wanted to thank you folks for your help, it's actually making more sense now :).
     
  15. Jun 18, 2013 #14
    If hope the sense your making is not based on QED Andrews post because it is incorrect.

    His example starts with x as number of people and y as number of apples, then at the end switches to y as the number of people and states that y (the number of people) is twice x (the number of apples)... which would be incorrect.

    What he meant to say is that if y is the number of apples, then y is twice x (the number of people) because each person has two apples.

    Or that if y is the number of apples, then x (the number of people) is one half of y because each person has two apples.
     
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