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    • Join Date: Jul 2006
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    #1

    Angry Bacteria

    , If a disinfectant achieves a 90% kill of a microbial population in 1 minute what is the minimum time required to kill 100% kill of 10,000 microbial cells in a test solution? Please help me to answer this question. Thanks
    Last edited by workm504; 12-Jul-2006 at 12:45. Reason: to get an answer faster.

  1. rewboss's Avatar

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    #2

    Re: Bacteria

    Um... you're asking a bit much from a language forum, aren't you?

    This place might be more appropriate to your needs, but please note that they take a dim view of people using the forum just to get answers for their homework.


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    #3

    Re: Bacteria

    Never mind whatever it's not homework idiot!!!!!!!!

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    #4

    Re: Bacteria

    There is, of course, no answer. This question contains a mathematical function that can be plotted as a curve that never touches zero. If you walked 90 percent of the distance from your present location to the nearest wall every minute, you would never reach the wall.

  2. #5

    Talking Re: Bacteria

    Also known as a "trick question".



    Chris
    http://www.hearseesay.com

  3. rewboss's Avatar

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    #6

    Re: Bacteria

    Quote Originally Posted by mykwyner
    There is, of course, no answer. This question contains a mathematical function that can be plotted as a curve that never touches zero. If you walked 90 percent of the distance from your present location to the nearest wall every minute, you would never reach the wall.
    Aha! You've fallen into the trap yourself. What do you suppose will happen when you have only one bacterium left? Can you kill 90% of a single bacterium?

    This fallacy is described in one of Zeno's paradoxes. He concluded that motion is impossible, because in order to get from A to B you first have to travel half the distance, then half the remaining distance, then half the distance still remaining after that, and so on for ever and ever. Thus you will always approach B, but you will never reach it.

    Life, however, tells us that Zeno was completely wrong, and so does mathematics if you work out the sums involved. In this case it's about the sum of the infinite series 1/2 + 1/4 + 1/8 +... which Zeno assumed to be infinity, but is actually 1.

    If it takes 1 minute to go 90% of the distance to the wall, work out the sum of the infinite series 0.9 + 0.45 + 0.225 +... and you'll find that the answer is 1, i.e. the distance from where you are to the wall. You will reach the wall after 1 minute and 6 and two-thirds seconds.

    Or imagine it this way: It takes you 1 minute to travel 90% of the distance. It then takes you a little less than 6 seconds to travel 90% of the remaining distance. Not only does the distance decrease with every step, so does the time required to travel that distance. At some point, it will take you an infinitesimal amount of time to travel an infinitesimal distance, and that's the point at which you hit the wall and say "ouch".

    Of course, the time required for each step remains constant in the original problem, but since you can only kill whole bacteria, it's not an issue. At some point there will be fewer than 10 bacteria left and the chances are they will all be dead after the next minute. I expect there's a simple formula to calculate this, but it is not a trick question.

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    #7

    Re: Bacteria

    Once you've got 1, then it would be dead the next minute surely, as the chemical kill bacteria, so it would be 10,000-1000-100-10-1-0, wouldn't it? If it didn't specify the number at the start, then we'd have the infinite cycle, but once one is left, it's going to die not be injured perpetually.

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