1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Probality Question

  1. Dec 13, 2008 #1
    1. The problem statement, all variables and given/known data
    A drinks machine dispenses coffee into cups. A ign on the machine inicats that each cup contains 50ml of coffee. The machine actually dispenses a mean amount of 55 ml per cup and 10% of the cups contain less than the amount stated on the sign. Assuming that the amount of coffee is normally distributed find

    a) the standard deviation of the amount of coffee dispensed per cup in ml.


    2. Relevant equations



    3. The attempt at a solution

    Well Z ~ N(u,o°)

    P(Z < 50) = 0.1

    standardise this

    [tex]P(Z < \frac{50-55}{o}) = 0.1[/tex]

    So what do I do next. I thought look up the prob distrubution of z = 0.1 in the prob dist tables = 0.5398 but the mark scheme doesn't show this happening at all.

    What should I do

    Thanks :)
     
  2. jcsd
  3. Dec 13, 2008 #2

    HallsofIvy

    User Avatar
    Staff Emeritus
    Science Advisor

    Looks like a good plan to me. But be careful of your numbers. You say you got P(0.1)= .5398 since 50 is less than the mean of 55, [itex]z= (50- 55)/\sigma[/itex] is negative and the probability must be less than 0.50. I suspect you are looking at a table of the normal distribution that only deals with positive values of z. Using the symmetry of the normal distribution, you want [itex]z= (50- 55)/\sigma = .5000- .5398= -.0398[/itex]. What is [itex]\sigma[/itex] for that?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Probality Question
  1. Proof Question (Replies: 2)

  2. Subset Question (Replies: 3)

  3. Probability Question (Replies: 12)

Loading...