Hey there! Sign in to join this conversationNew here? Join for free
    • Thread Starter
    Offline

    0
    ReputationRep:
    The question states:

    Two events R and S are independant. P(R|S) = 3/4 AND P(S) = P(S'nR'). By letting x equal P(S) and forming an equation for x, or otherwise, find:

    a) P(S)
    b) P(S'UR)
    c) Write down P(S|R)


    There's obviously some link between P(S) = P(S'nR') and R and S's indepedence but I'm not seeing it.

    Help.

    Socratic method preferred.
    • Study Helper
    Offline

    13
    (Original post by Mambo No. 5)
    ...
    Two hints that will help you.

    1) What's P(R)?

    2) If S and R are independent, what can you can about S' and R'?
    • Thread Starter
    Offline

    0
    ReputationRep:
    (Original post by ghostwalker)
    Two hints that will help you.

    1) What's P(R)?

    2) If S and R are independent, what can you can about S' and R'?
    1) ?

    2) Well if they're independent, then we know that P(S) x P(R) = P(SnR)
    • Thread Starter
    Offline

    0
    ReputationRep:
    (Original post by ghostwalker)
    Two hints that will help you.

    1) What's P(R)?

    2) If S and R are independent, what can you can about S' and R'?
    I just put:

    P(R) x P(S)/P(S) = 3/4

    Cancelling out P(S) means P(R) = 3/4 ?
    • Study Helper
    Offline

    13
    (Original post by Mambo No. 5)
    1) ?
    You're told P(R|S) and that R and S are independent. Use the definition of independence.


    2) Well if they're independent, then we know that P(S) x P(R) = P(SnR)
    What about S' and R' ?
    • Study Helper
    Offline

    13
    (Original post by Mambo No. 5)
    I just put

    P(R) x P(S)/P(S) = 3/4

    Cancelling out P(S) means P(R) = 3/4 ?
    Yes.
    • Thread Starter
    Offline

    0
    ReputationRep:
    (Original post by ghostwalker)
    Yes.
    So if P(R) is equal to 3/4


    ...How is this fact used with P(S'nR') to find P(S)?


    I know P(R') = 1/4 if that's relevant.
    • Thread Starter
    Offline

    0
    ReputationRep:
    Bump.
    • Thread Starter
    Offline

    0
    ReputationRep:
    Bump.
    • Thread Starter
    Offline

    0
    ReputationRep:
    Bump.
    • Study Helper
    Offline

    13
    (Original post by Mambo No. 5)
    So if P(R) is equal to 3/4


    ...How is this fact used with P(S'nR') to find P(S)?


    I know P(R') = 1/4 if that's relevant.
    You keep ignoring my question about R' and S'; so much for Socrates.

    P(R'nS') = P(R')P(S') since R and S are independent, then so are R' and S'. Using that you can formulate your equatiion in "x" as per the original question.
    • Thread Starter
    Offline

    0
    ReputationRep:
    (Original post by ghostwalker)
    You keep ignoring my question about R' and S'; so much for Socrates.

    P(R'nS') = P(R')P(S') since R and S are independent, then so are R' and S'. Using that you can formulate your equatiion in "x" as per the original question.
    So far I've got:

    P(R|S) = 3/4
    P(R) = 3/4
    P(R') = 1/4

    P(SnR) = P(S) X P(R)
    P(S'nR') = P(S') X P(R')

    P(SnR) = P(S) X 3/4
    P(S'nR')= P(S') X 1/4

    P(R'|S') = 1/4

    (Mind Block)
    • Study Helper
    Offline

    13
    (Original post by Mambo No. 5)
    So far I've got:

    P(R|S) = 3/4
    P(R) = 3/4
    P(R') = 1/4

    P(SnR) = P(S) X P(R)
    P(S'nR') = P(S') X P(R')

    P(SnR) = P(S) X 3/4
    P(S'nR')= P(S') X 1/4

    P(R'|S') = 1/4

    (Mind Block)
    Use the method they suggest in the question on the equation P(S) = P(S'nR'). You've worked out most of the stuff you need for it.
    • Thread Starter
    Offline

    0
    ReputationRep:
    (Original post by ghostwalker)
    Use the method they suggest in the question on the equation P(S) = P(S'nR'). You've worked out most of the stuff you need for it.
    That's (^^^) the part I seem to be having the most trouble with.


    ......Thank you for your patience.
    • Study Helper
    Offline

    13
    (Original post by Mambo No. 5)
    That's (^^^) the part I seem to be having the most trouble with.


    ......Thank you for your patience.
    The only bit you are missing is P(S') = 1- P(S)

    You now have everything you need to make the equation using the substitution they suggest.

    Post working if you're still having problems.
 
 
 
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • Poll
    What newspaper do you read/prefer?
    Useful resources

    Make your revision easier

    Maths

    Maths Forum posting guidelines

    Not sure where to post? Read the updated guidelines here

    Equations

    How to use LaTex

    Writing equations the easy way

    Student revising

    Study habits of A* students

    Top tips from students who have already aced their exams

    Study Planner

    Create your own Study Planner

    Never miss a deadline again

    Polling station sign

    Thinking about a maths degree?

    Chat with other maths applicants

    Can you help? Study help unanswered threads

    Groups associated with this forum:

    View associated groups
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

    Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

    Quick reply
    Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.