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!

Homework Help: Maths statistic(discrete)

  1. Jul 21, 2010 #1
    I am having problem to calculate some probability...please help me and explain to me how you get the answer

    In order to use the labels in a competition a housewife removes the labels from 3 tins of tomato soup and from 4 tins of peaches, but forget to mark the tins which, without their labels are all identical. The housewife opens successive tins, chosen at random, looking for a tin of peaches. The random variable X denotes the number of tins opened up to and including the first tin of peaches.

    Tabulate the probability distribution of X and show that var(X) = 16/25

    Please help me i just don't know where to start..
  2. jcsd
  3. Jul 21, 2010 #2
    Question says we must draw up a probability distribution table of X = the number of tins one must open to get a peach. So lets start at that...

    Say X = 1. So, we need to find the probability that in the very first try we open a peach tin. Can you find this?

    When X = 2, we need the probability that the first tin we opened was NOT peach, but the second tin was.

    For X = 3, find probability that the first two tins opened were tomatos, and the third one was peach...and so on

    Similarly, you find all the probabilities, till X = 4 (Why 4??)

    Then draw a table. That is your answer. Then find var(X) with the formula.
  4. Jul 22, 2010 #3

    Hmmmm i understand it partially. But i think that if my table is correct that is the total probability = 1 so i can manage the variance

    for the probability of of getting 1 peach tin

    P(x=1) = (4/7)

    Getting it on the 2nd trial
    P(x=2) = ( 3/7 x 4/6) = (2/7)

    Getting it on the 3th trial
    P(x=3) = ( 3/7 x 2/6 x 4/5) = (4/35)

    Getting it on the fourth trial
    P(x=4) = ( 3/7 x 2/6 x 1/5 x 4/4) = (1/35)

    SO adding all the probability give me 1 (So my answer must be correct..hehehe
  5. Jul 22, 2010 #4
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook