Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Cumulative probability continued

  1. Jun 7, 2010 #1
    cumulative probability continued....

    Ok. I have 25 gumball machines with red or green gumballs. Each machine has a different percentage of each. Number 1 has 70% red, 30% green. Number 2 has 95% red, 5% green, and so on. I buy 1 gumball from each an drop it into a bag. Then reach into the bag and pull a random gumball. How to calculate probability Red or Green from the bag?
  2. jcsd
  3. Jun 7, 2010 #2


    User Avatar
    Science Advisor
    Homework Helper

    Re: cumulative probability continued....

    I would start calculating the expectation value of the green and red balls in the bag.

    For example, if you draw one from each machine, you have on average (after repeating this experiment very very very often), (0,70 + 0,95 + ...) green balls in the bag, and (0,30 + 0,05 + ....) red ones.

    Let's assume for this example that there are only two machines, so you will have 1,65 green ball and 0,35 red balls in a total of two.
    If you draw one of the balls randomly, therefore, the probability of picking the green one is 1,65 / 2 = 0,825 and for the red one of course 0,175. (If you go back and check the calculation, this is basically the average of the probabilities, because we drew equal numbers of balls from each machine).
  4. Jun 14, 2010 #3
    Re: cumulative probability continued....

    Thanks for the reply.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook