A maths question

Discussion in 'Community Discussion' started by Nermal, Jan 26, 2007.

  1. Nermal Moderator

    Nermal

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    #1
    Car A is travelling at 25 km/h. Car B is going 3x faster. How fast is Car B travelling?
     
  2. mkrishnan Moderator emeritus

    mkrishnan

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    #2
    Erm... is this a mathematics question or a semantics question? That is, is the active question the difference between "going three times as fast as" and "going three times faster than"?

    Typically, at least to an American Engineering-trained mind... :D the latter phrase should be deprecated.

    That is, I typically say:

    A is 3x as fast as B -- in this example, 75 km/h

    But I don't typically say

    A is 3x faster than B
    or
    A is 300% faster than B

    Because it's never clear if the desired answer is 75 km/h or (what I feel is more technically correct) 100 km/h.
     
  3. tobefirst macrumors 68040

    tobefirst

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  4. SilentPanda Moderator emeritus

    SilentPanda

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    #4
    3x faster!

    At first guess I would actually say 75 km/hr. Although I guess it could be 100 km/hr as brought up above. Most sane people would say 75 km/hr I believe.
     
  5. applelabs macrumors member

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    #5
    OK, I'll make a fool of myself...

    Assuming you mean can B is moving 3x (three times) faster than car A....
    25 km/h x 3 = 75 km/h

    So car B is moving at 75 km/h.
     
  6. phungy macrumors 68020

    phungy

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    #6
    The answer is D, Not enough information given. :cool:

    That is what I had initially thought but I'm sure the question wasn't worded correctly.
     
  7. applelabs macrumors member

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    #7
    Ahh.... Of course. I feel so stupid.

    You have to tell us the color/s of each car.
     
  8. Nermal thread starter Moderator

    Nermal

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    #8
    Basically I was looking to see whether you guys answered 75 or 100, and I got a mixture of both, which is what I expected.

    The reason I bring this up is because Apple uses what I believe to be incorrect notation on their site. For example, the new N AirPort says "5x faster" in some places but "5x the performance" in others; surely 5x faster means 6x the performance.
     
  9. mkrishnan Moderator emeritus

    mkrishnan

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    #9
    See, that's my point, though... traditionally, people *do* say things like "A is 100% faster than B" or "A is 50% faster than B." Both of these statements are clearly meant to be interpreted as implying A is faster than B. They are used commonly and almost invariably in this way.

    Specifically, for the car B traveling at 25 km/h

    A is 100% faster than B is almost always interpreted to mean that A = 50 km/h
    A is 50% faster than B is almost always interpreted to mean A = 37.5 km/h

    This latter phrase is never interpreted as A = 12.5 km/h.

    So it follows that

    A is 300% faster than B should mean A = 100 km/h
    and the identical statement:
    A is 3x faster than B should mean A = 100 km/h as well.

    Which, again, is why I avoid saying this. I think most "trained" or formal users of applied math -- engineers, etc. -- will expect you to mean this when you use this kind of terminology. But I agree that how people who are not professionals will make mixed interpretations.

    And now that Nermal tipped his hand, I see that's what he was getting at. :p
     
  10. killmoms macrumors 68040

    killmoms

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    #10
    I tend to ignore the words following 5x, as that tells me all I need to know: multiply the old result by 5, get new result. The math of 5x is always the same—I prefer not living in some bizarre world where 5 = 6. ;)

    EDIT: Well, no, because they're using percentages faster. When someone says 50% faster, they mean 1.5x as fast (since they're essentially saying it's as fast as the original PLUS half that again). I dislike the old "N% faster" terminology, and prefer straight 1.5x, 2.2x, whatever, since then it's always clear what is meant.
     
  11. mkrishnan Moderator emeritus

    mkrishnan

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    #11
    300% and 3x are the same by definition... at least to me. I feel that most engineers -- the people who actually design the technology that is 5x as fast as whatever -- would agree with that statement. The key word is the one you ignore -- "faster" vs. "as fast." That's exactly why I'm saying I hate using the 3x or 300% faster than. ;)
     
  12. yellow Moderator emeritus

    yellow

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  13. tobefirst macrumors 68040

    tobefirst

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    #13
    For the record, if I didn't have any reason to believe that there was something more going on than just a simple question (like, if my nephew had asked me while working on homework, for instance) my answer would have been 75.
     
  14. nitynate macrumors 6502a

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    #14
    Wait....

    The answer is obvious, it is 100.

    Durrr.
     
  15. tobefirst macrumors 68040

    tobefirst

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    #15
    The only thing obvious is that the answer is not so obvious. Otherwise half the people wouldn't have said 75. :)
     
  16. killmoms macrumors 68040

    killmoms

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    #16
    I think we agree, in a roundabout way.

    In fact, lemme boil our argument down into something we can both easily agree on:

    Stupid ambiguous English! :D
     
  17. dernhelm macrumors 68000

    dernhelm

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    #17
    Are we all so certain that the question is actually grammatically correct in the first place?
     
  18. applelabs macrumors member

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    #18
    No, there is an extra "l" in "traveling".
    Besides that, I think it is correct.

    From what mkrishnan is saying (I think).
    Car B is going 3x faster [than car A].
    That means car B's speed equals [(speed of car A + (three times speed of car A)]
     
  19. bearbo macrumors 68000

    bearbo

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    #19
    no, (as an engineer too), i think
    - 300% faster means (as example of 50% faster, means (0.5+1)x) 300%faster aka (3+1)x = 4x

    -however when 3x means 3 times of, which implies whatever it is, is 300% of the previous thing.
    edit: however the "3x faster" is controdicting because 3x implies 3 "times of", and followed by "faster" which means 3+1 times of

    i do agree with you, tho, that the wording is very ambiguous.
     
  20. nitynate macrumors 6502a

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    #20
    Well, the reason many people said 75 is because they arent making the equation in thier head correctly, or i could be wrong....
    I just made 25*3+(12.5*2) in my head and got 100, which is the answer to 3x faster.
    [because 12.5 is half of 25 and we are adding the number twice more]

    I think.
    100 just came to my mind when i did it in my head, i could be wrong.


    But i'm not wrong. i am good at math. ahhh! be more specific in your questions.
     
  21. bearbo macrumors 68000

    bearbo

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    #21
    enlighten me why you do the 12.5 part?
     
  22. bearbo macrumors 68000

    bearbo

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  23. phungy macrumors 68020

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    #23
    That's what I was wondering when I first saw the thread, heh.
     
  24. nitynate macrumors 6502a

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    #24
    Because we are making it 3x as much, which will require half of the original in each number we add to get our final answer.

    Isnt that how *x problems work?
     
  25. bearbo macrumors 68000

    bearbo

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    #25
    no offend to you, nitynate, but is it just me that has no idea what that means?

    when i learend "3x as much" (and that's not even the 3x faster which is very ambiguous).. say a is "3x as much" of b, then a = 3b .. and if b = 25 as in this case, a = 3*25 = 75 (again, i'm talking about 3x as much, not 3x faster in the original post to illustrate how i learnt "3x as much")
    but that doesn't involve b/2 anywhere in that equation...
     

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