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Thread: Normal variable day-week multiplication?

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    Normal variable day-week multiplication?




    Hello Statisticians!

    I would really appreciate help with this:

    The mean number of lemonade glasses sold in a day is 15, standard deviation is 10. Find the probability of selling at least 120 glasses in 7 days (1week)?

    The correct answer given in the book is 0.28.

    My attempt: 1 - Z<120-(15x7)/(10x7) but I'm getting 0.4.

    Thank you in advance. Xx

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    Re: Normal variable day-week multiplication?

    You probably wanted 1 - P(Z < ...). You need to be careful and frame it as a probability - what you have is just a set.

    The other issue you might want to ask yourself is if the standard deviation of the sum of 7 random variables is just 7 times the standard deviation. Hint: it isn't.
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    Re: Normal variable day-week multiplication?


    Thanks for your reply.

    Yes I meant P(Z<..), thanks for pointing that out.

    Well I figured what I'm doing is wrong, but I can't figure what the right way is.

    My question in general is: how do we work out the probability of multiple trials where each follows a normal distribution?

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