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: 2D probability question

  1. Feb 2, 2010 #1
    we throw 3 coins

    y represents the throw of three coins
    x represents the throw of the first two
    we count the number of hetz gotten from the thrown coins.

    the solution says for the slot that:
    but i cant understand how
    the logical solution says:
    for x to be 1 we have 01 10
    so for y to be 2 011 101
    so with respect to y we have two possibilities from 8

    but the formal mathematical says

    for P(x=1) we need to have 01 10 which is 1/2 or 0.5
    for P(y=2/x=1) its 2/8 or 0.25

    and not 0.25
  2. jcsd
  3. Feb 2, 2010 #2
    looks like the only mistake is in this last line. p(y=2|x=1) would be 1/2

    the "logical" solution is fine, and the best way to do the problem.
  4. Feb 2, 2010 #3
    why p(y=2|x=1) 1/2 ?
    3 digit gives 8 possibilities
    our possibilities are 011 101
    so its 1/4
  5. Feb 2, 2010 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Education Advisor

    You're mixing up p(y=2 and x=1) and p(y=2|x=1). The probability p(y=2 and x=1) is 2/8, but p(y=2|x=1) = p(y=2 and x=1)/p(x=1) = (2/8)/(1/2) = 1/2.

    Intuitively, when you're given x=1, the possible outcomes are restricted to 010, 011, 100, and 101, so you divide by 4 instead of 8.
  6. Feb 3, 2010 #5
    thanks i understand now
    Last edited: Feb 3, 2010
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook