1. Limited time only! Sign up for a free 30min personal 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!

Probability question

  1. Jan 8, 2009 #1
    Let's say we have 10 boxes and I open each of them one by one...

    I open the 1st box, there is a toy car in it.
    I open the 2nd box, there is also a toy car in it.
    I open the 3rd box, there is also a toy car in it.
    I open the 9th box, there is also a toy car in it. :) wow, I got 9 toy cars in 9 boxes...

    what is the probability that 10th box also has a toy car in it?

    also generalize 10 to any number...
    what is the probability that nth box also has a toy car in it, if all n-1 boxes each have a toy car in them.
  2. jcsd
  3. Jan 8, 2009 #2


    User Avatar
    Science Advisor
    Homework Helper

    There are lots of ways to do this binomial confidence interval problem. One common way is prob = (# successes + 1) / (# trials + 2), which would suggest a 91% chance.
  4. Jan 9, 2009 #3
    Depends on where you're getting the boxes.
  5. Jan 9, 2009 #4


    User Avatar
    Science Advisor

    Yes this is a silly question, unless you give some more information there is no well defined answer.

    About the best answer I could give (without any additional information) would be,

    [tex]P = \frac{m-9}{n-9}[/tex]

    Where n is the number of "boxes" in the universe and m<n is number of boxes in the universe that contain toy cars. I know that's not a very useful answer, but you know that if you want a useful answer you have to ask a sensible question right.
  6. Jan 10, 2009 #5


    User Avatar
    Science Advisor

    What CRGreathouse is suggesting is to use the sample data to estimate the probability that a single box contains a car. Of course, if you have gotten a car in every box so far, the "maximum likelihood" estimate of that probability is 1 but you can use the sample size to put bounds on a confidence interval for it.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook