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Relation between values in %

  1. Nov 29, 2016 #1
    I have a really simple question that I can't figure it out.

    I have a process X that took 905 seconds to finish. Process Y took 950 seconds to finish. I want to know a) how much in % process X is faster than Y, and b) how much in % process Y is slower than X?

    I think that the answer is the following.

    a) Taking X as reference, X is faster (X-Y)/Y times.
    b) Taking Y as reference, Y is slower 1-(Y/X) times.

    Am I correct?
     
  2. jcsd
  3. Nov 29, 2016 #2

    haruspex

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    That depends what you mean by X and Y there.
    Since you want how much faster one is than the other, you need to compare rates.
    What are the rates? You can express them as "processes per second"
     
  4. Nov 29, 2016 #3
    My question is not related to processes per second.
    I have a process X which is faster than Y. I want to know how much X is faster than Y in %, and how much Y is slower than X in %. Eg., X is faster 10% than Y, and Y is slower something in % than X.
     
    Last edited: Nov 29, 2016
  5. Nov 29, 2016 #4

    haruspex

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    Alice walked a km in 15 minutes, Bob took 30 minutes. Alice was faster by 15 minutes, which is 50% less than 30, but was she only 50% faster? At 4km/h, wasn't she 100% faster than Bob's 2km/h?
    "% faster" means you are comparing rates, not times.
     
  6. Nov 29, 2016 #5
    I understand your example because you use numbers easy to calculate. I don't know how to answer my example. Can you tell me if my answers are correct?
     
  7. Nov 29, 2016 #6

    Mark44

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    Using your numbers, for (a), you would get (905 - 950)/950, which is a negative number.
    For (b), you would get 1 - (950/900), which is a number smaller than -1; i.e., more negative than -1.
    So no, these are both incorrect.

    See if you can duplicate @haruspex's thinking using your numbers.
     
  8. Nov 29, 2016 #7
    I am having some difficulty in duplicating @haruspex's example. I really need some help.
     
  9. Nov 29, 2016 #8

    haruspex

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    Do as I suggested at the start - express the two rates as numbers of processes per second.
     
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