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

    0
    ReputationRep:
    Name:  Screen Shot 2013-05-14 at 09.41.52.png
Views: 182
Size:  50.0 KB

    This is from a Delphis paper- I am quite confused at question b). Surely if the card is TRANSFERRED from Group A to group B then the probabilities would be:

    (5/29) * (7/32)?

    Yet the mark scheme says:

    (5/30 * (8/32), implying that the number of cards in pack B has increased, but not decreased in A. Also implies that the card from the question has first digit 1, when it is actually first digit 2?

    I'm probability just being really dense but does any one have any idea what to do here?
    Thanks, much appreciated.
    • Study Helper
    Offline

    16
    ReputationRep:
    Study Helper
    (Original post by lindseyyy)
    Name:  Screen Shot 2013-05-14 at 09.41.52.png
Views: 182
Size:  50.0 KB

    This is from a Delphis paper- I am quite confused at question b). Surely if the card is TRANSFERRED from Group A to group B then the probabilities would be:

    (5/29) * (7/32)?

    Yet the mark scheme says:

    (5/30 * (8/32), implying that the number of cards in pack B has increased, but not decreased in A. Also implies that the card from the question has first digit 1, when it is actually first digit 2?

    I'm probability just being really dense but does any one have any idea what to do here?
    Thanks, much appreciated.
    Can you post the table of information that goes with this question?
    • Thread Starter
    Offline

    0
    ReputationRep:
    (Original post by davros)
    Can you post the table of information that goes with this question?

    Oh yeah, sorry! Name:  New SS.png
Views: 88
Size:  27.7 KB
    • Study Helper
    Offline

    16
    ReputationRep:
    Study Helper
    (Original post by lindseyyy)
    Name:  Screen Shot 2013-05-14 at 09.41.52.png
Views: 182
Size:  50.0 KB

    This is from a Delphis paper- I am quite confused at question b). Surely if the card is TRANSFERRED from Group A to group B then the probabilities would be:

    (5/29) * (7/32)?

    Yet the mark scheme says:

    (5/30 * (8/32), implying that the number of cards in pack B has increased, but not decreased in A. Also implies that the card from the question has first digit 1, when it is actually first digit 2?

    I'm probability just being really dense but does any one have any idea what to do here?
    Thanks, much appreciated.
    Event A_{1} refers to the original situation when you take a card from pack A and look at its first digit - so you're looking at the original number of cards in A. If you now put this card in B and look at another card in A, then A is now smaller!
    • Thread Starter
    Offline

    0
    ReputationRep:
    (Original post by davros)
    Event A_{1} refers to the original situation when you take a card from pack A and look at its first digit - so you're looking at the original number of cards in A. If you now put this card in B and look at another card in A, then A is now smaller!
    Yes but the value for A isn't smaller! It's the same?
    • Study Helper
    Offline

    16
    ReputationRep:
    Study Helper
    (Original post by lindseyyy)
    Yes but the value for A isn't smaller! It's the same?
    I think you're confusing yourself about what's happening here (or confusing me )

    We take a card from A. Its first digit might be 1, with probability 5/30; or it might be 2, with probability 7/30; or it might be one of the other possibilities.

    When they say "write down P(A_2) ", they're just asking you to write down the relevant probability - they're NOT telling you that the first digit is 2.

    Now the card is put in pack B (so B is larger) and we draw a card from B. Its first digit could be 1, with probability B_1, or one of the other probabilities.

    So when they ask you about P(A_1 \cap B_1) they're asking you to combine the probability from the 1st draw from A with the probability from the 1st draw from B. You're not taking another card from the (smaller) pack A!

    Does this help?
    • Thread Starter
    Offline

    0
    ReputationRep:
    (Original post by davros)
    I think you're confusing yourself about what's happening here (or confusing me )

    We take a card from A. Its first digit might be 1, with probability 5/30; or it might be 2, with probability 7/30; or it might be one of the other possibilities.

    When they say "write down P(A_2) ", they're just asking you to write down the relevant probability - they're NOT telling you that the first digit is 2.

    Now the card is put in pack B (so B is larger) and we draw a card from B. Its first digit could be 1, with probability B_1, or one of the other probabilities.

    So when they ask you about P(A_1 \cap B_1) they're asking you to combine the probability from the 1st draw from A with the probability from the 1st draw from B. You're not taking another card from the (smaller) pack A!

    Does this help?


    Thanks for the help! I get it but it's still annoyingly confusing- I am just going to pray that anything remotely like this doesn't come up in the exam tomorrow!
    • Study Helper
    Offline

    16
    ReputationRep:
    Study Helper
    (Original post by lindseyyy)
    Thanks for the help! I get it but it's still annoyingly confusing- I am just going to pray that anything remotely like this doesn't come up in the exam tomorrow!
    No problem. Good luck with the exam
    Offline

    1
    ReputationRep:
    :/ do we actually get qs like this in the exam?
 
 
 
    Discussions on TSR

  • Latest
  • Personalise
Poll
Do you agree with the PM's proposal to cut tuition fees for some courses?
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

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

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