Hey there! Sign in to join this conversationNew here? Join for free

Stuck on question (Probability or possible quadratic equation) Watch

    • Thread Starter
    Offline

    8
    ReputationRep:
    Hi .

    Not sure how to do this question. It mentioned probability but am I to just solve it like a quadratic equation?

    Thanks
    Attached Images
     
    Online

    22
    ReputationRep:
    (Original post by jojo55)
    Hi .

    Not sure how to do this question. It mentioned probability but am I to just solve it like a quadratic equation?

    Thanks
    No. Draw a tree diagram. What is the probability that Joshua takes one white sock and one black sock in terms of y?

    Equate this expression to 6/11 and simplify/re-arrange to get the given quadratic equation. They do not want you to solve it just yet, they want you to use the facts given to get that quadratic.
    • Thread Starter
    Offline

    8
    ReputationRep:
    (Original post by Zacken)
    No. Draw a tree diagram. What is the probability that Joshua takes one white sock and one black sock in terms of y?

    Equate this expression to 6/11 and simplify/re-arrange to get the given quadratic equation. They do not want you to solve it just yet, they want you to use the facts given to get that quadratic.


    I get \frac{y}{y+5} to show the probability of a white sock out of the total amount of socks of y + 5

    Plus, I get \frac{5}{y+5} to show the probability of a black sock.

    Is the next bit:

    (\frac{y}{y+5})(\frac{5}{y+5}) = \frac{6}{11}

    Not 100% on that as the mark scheme says it is 2(\frac{y}{y+5})(\frac{5}{y+5}) = \frac{6}{11} instead.
    Online

    22
    ReputationRep:
    (Original post by jojo55)
    I get \frac{y}{y+5} to show the probability of a white sock out of the total amount of socks of y + 5

    Plus, I get \frac{5}{y+5} to show the probability of a black sock.

    Is the next bit:

    (\frac{y}{y+5})(\frac{5}{y+5}) = \frac{6}{11}

    Not 100% on that as the mark scheme says it is 2(\frac{y}{y+5})(\frac{5}{y+5}) = \frac{6}{11} instead.
    You've done:

    P(white sock) * P(black sock).

    What about the other possibility of:

    P(black sock) * P(white sock)?

    You need to add those together:

    P(white sock)*P(black sock) + P(black sock)*P(white sock) = \frac{y}{y+5} \cdot \frac{5}{y+5} + \frac{5}{y+5} \cdot \frac{y}{y+5}

    Did you draw a tree diagram? If so, it should have been obvious of the two different routes you can go to get the requires probability.
    Offline

    6
    ReputationRep:
    (Original post by jojo55)
    Hi .

    Not sure how to do this question. It mentioned probability but am I to just solve it like a quadratic equation?

    Thanks
    took me a while but i think i got it lol:

    2(y/y+5 x 5/y+4) = 6/11

    2(5y/y2 + 9y + 20) = 6/11

    10y/y2 + 9y + 20 = 6/11

    110y/y2 + 9y + 20 = 6

    110y = 6y2 + 54y + 120

    0= 6y2 - 56y + 120

    0= 3y2 -28y +60
    Online

    22
    ReputationRep:
    (Original post by qwerty0301)
    took me a while but i think i got it lol:
    Full solutions are against forum guidelines, have you read the posting guide?

    Thanks.
    • Thread Starter
    Offline

    8
    ReputationRep:
    (Original post by Zacken)
    Full solutions are against forum guidelines, have you read the posting guide?

    Thanks.
    (Original post by qwerty0301)
    took me a while but i think i got it lol:

    2(y/y+5 x 5/y+4) = 6/11

    2(5y/y2 + 9y + 20) = 6/11

    10y/y2 + 9y + 20 = 6/11

    110y/y2 + 9y + 20 = 6

    110y = 6y2 + 54y + 120

    0= 6y2 - 56y + 120

    0= 3y2 -28y +60

    Thanks for the help.

    I got the same solution after I figured out that that it wasn't both over y+5, that I put in my earlier post.

    Thanks again.
 
 
 
  • 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
    Would you like to hibernate through the winter months?
    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.