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Kapaln math #22

Discussion in 'DAT Discussions' started by Greensalad, Apr 18, 2007.

  1. Greensalad

    10+ Year Member

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    22. If a diameter of a cylinder is increased by 20% and the height is decreased by 16 2/3 %, by what percent is the volume changed?
    A. -10
    B. 10
    C. 20
    D. 24
    E. 83 1/2

    Answer is C

    I can do this problem but I would need more than a minute. What's the quickest way to do it?
     
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  3. Greensalad

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    Alright I can do it in 40 seconds if I plug in numbers, but I knew which numbers to plug in before I started doing the problem. I don't think plugging in numbers is a good idea.
     
  4. infernobutterfl

    infernobutterfl Pyro user
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    We are supposed to do the math out by hand? (possible, but that is just a waste of time and effort.) if only the 24% was not there, then the answer would have been much easier. (one could guess-timate the answer)
     
  5. Streetwolf

    Streetwolf Ultra Senior Member
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    Volume = pi*r^2*h
    New Volume = pi*(1.2r)^2*(0.833h)

    We write 1.2 as (6/5) and 0.833 as (5/6).

    So we have New Volume = pi*(6/5)^2*r^2*(5/6)*h
    New Volume = (6/5)pi*r^2*h

    The new volume is 1.2 * the old volume, so the new volume is increased by 20%.

    C.
     
  6. Greensalad

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    Thanks. Your QR's probably a 30.
     
  7. Streetwolf

    Streetwolf Ultra Senior Member
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    28. Dunno if that was perfect or not :p
     

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