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Math Help - Discrete or continuous?

  1. #1
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    Discrete or continuous?

    The shelf life, in days, for bottles of a certain medicine is a random variable that has the density function

    <br />
f(x) = \frac{20000}{(x+100)^3}, x > 0
    0, elsewhere

    What is the probability that a bottle of this medicine will a shell life of
    atleast 200 days?

    My professor said that this is a continuous random variable and gave this integral:  \int^{\infty}_{200} \frac{20000}{(x+100)^3} dx  =<br />
\frac{-10000}{(x+100)^2} from 200 to infinity

    I say the random variable is discrete because the number of days is countable.
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  2. #2
    MHF Contributor matheagle's Avatar
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    It's certainly a continous random variable.
    That is a valid density.
    However, the number of days of anything should be a discrete rv.
    Hence that density may not be appropriate in this setting.
    To make you both winners it should say that the distribution of days is APPROXIMATELY....
    Last edited by matheagle; March 10th 2010 at 11:48 PM.
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