+ Reply to Thread
Results 1 to 10 of 10

Thread: red balls and blue balls

  1. #1
    Points: 65, Level: 1
    Level completed: 30%, Points required for next Level: 35

    Posts
    6
    Thanks
    0
    Thanked 0 Times in 0 Posts

    red balls and blue balls




    Hi there everyone

    I'm a complete newbie to probability and currently taking a foundation degree at Uni of which statistics is one of my modules. I have a mock paper here and one of the questions is as follows:

    A bag contains 6 red balls and 4 blue balls. One ball is taken out. What is the probability that I will pick 2 blue balls?

    If anyone could explain how one works this out I will be extremely grateful

    tim

  2. #2
    Omega Contributor
    Points: 38,334, Level: 100
    Level completed: 0%, Points required for next Level: 0
    hlsmith's Avatar
    Location
    Not Ames, IA
    Posts
    6,998
    Thanks
    398
    Thanked 1,186 Times in 1,147 Posts

    Re: red balls and blue balls

    Quote Originally Posted by timlazisa View Post
    One ball is taken out. What is the probability that I will pick 2 blue balls?

    tim
    So when a ball is taken out, now our denominator will be one less. However they do not tell us what color of a ball was removed. They also don't tell use if after selection of our first ball, whether they replace it. They also don't tell us how many tries to get two blue balls, just two selections? Just to confirm, as written, this is exactly how it was presented?
    Stop cowardice, ban guns!

  3. #3
    Points: 65, Level: 1
    Level completed: 30%, Points required for next Level: 35

    Posts
    6
    Thanks
    0
    Thanked 0 Times in 0 Posts

    Re: red balls and blue balls

    Hi

    Thank you so much for replying, I have this one sorted now. However I need loads of help. This next question has me beat:

    The marks for a national maths test are normally distributed with a mean of 60 and a standard deviation of 15

    Question 1) a child has a mark of 70. using normal distribution calculate the probability:- P(m<70)

    Question 2) Same question for a child with 82:- P(m<82)

    Question 3) What is the probability that a third child taken at random has a mark between 70 and 82?

    Please explain like I'm 5 years old here. i'm 52 just started a degree and this stuff beats me?!

    many thanks

    Tim

  4. #4
    Points: 3,621, Level: 37
    Level completed: 81%, Points required for next Level: 29
    askazy's Avatar
    Location
    Holand
    Posts
    160
    Thanks
    4
    Thanked 0 Times in 0 Posts

    Re: red balls and blue balls

    You know how to use the table of the standard normal distribution?
    The difference between stupidity and genius is that genius has its limits.
    "Albert Einstein"

  5. #5
    Points: 65, Level: 1
    Level completed: 30%, Points required for next Level: 35

    Posts
    6
    Thanks
    0
    Thanked 0 Times in 0 Posts

    Re: red balls and blue balls

    Hi

    yes I have this

  6. #6
    Points: 3,621, Level: 37
    Level completed: 81%, Points required for next Level: 29
    askazy's Avatar
    Location
    Holand
    Posts
    160
    Thanks
    4
    Thanked 0 Times in 0 Posts

    Re: red balls and blue balls

    Quote Originally Posted by timlazisa View Post
    Hi

    yes I have this
    Look, the key to solving the first two exercises is to transform the random variable "m" in a normal standard.

    If m is N(\mu,\sigma^2)\rightarrow \frac{m-\mu}{\sigma} is N(0,1). So P(m<82)=P(\frac{m-\mu}{\sigma}<\frac{82-\mu}{\sigma}). Denoting by Z the standard normal random variable P(Z<\frac{82-60}{15})=P(Z<\frac{22}{15})=P(Z<1.46)
    Now just look the value in the standard normal table.
    The difference between stupidity and genius is that genius has its limits.
    "Albert Einstein"

  7. #7
    Points: 65, Level: 1
    Level completed: 30%, Points required for next Level: 35

    Posts
    6
    Thanks
    0
    Thanked 0 Times in 0 Posts

    Re: red balls and blue balls

    Dude thanks you. I've concluded that the answer sheet for this test paper is full of errors. The answers are incorrect. Thank you so much for your help. Is it ok to contact you again if I get stuck?

  8. #8
    Points: 3,621, Level: 37
    Level completed: 81%, Points required for next Level: 29
    askazy's Avatar
    Location
    Holand
    Posts
    160
    Thanks
    4
    Thanked 0 Times in 0 Posts

    Re: red balls and blue balls

    Quote Originally Posted by timlazisa View Post
    Dude thanks you. I've concluded that the answer sheet for this test paper is full of errors. The answers are incorrect. Thank you so much for your help. Is it ok to contact you again if I get stuck?
    I recommend that you try again, but if you can not, post your question in the same, that I or someone to help you.
    The difference between stupidity and genius is that genius has its limits.
    "Albert Einstein"

  9. #9
    Points: 65, Level: 1
    Level completed: 30%, Points required for next Level: 35

    Posts
    6
    Thanks
    0
    Thanked 0 Times in 0 Posts

    another one

    having difficulty with this one as well, again tutor just skimmed over it and I don't understand.
    Attached Images

  10. #10
    Points: 3,621, Level: 37
    Level completed: 81%, Points required for next Level: 29
    askazy's Avatar
    Location
    Holand
    Posts
    160
    Thanks
    4
    Thanked 0 Times in 0 Posts

    Re: red balls and blue balls


    I think this is a typical exercise of conditional probability.

    If E and F are two events, so

    P(E|F) = \frac{P(EF)}{P(F)}=\frac{P(E \cap F)}{P(F)} since P(F)>0

    And remember the formula of Bayes
    If E is an event, so
    P(E)=P(EF)+P(EF^c)=P(E|F)P(F)+P(E|F^c)[1-P(F)]

    If you do not understand I recommend you take a look at "A first course in probability, Sheldon Ross" in Chapter 3
    The difference between stupidity and genius is that genius has its limits.
    "Albert Einstein"

+ Reply to Thread

           




Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts






Advertise on Talk Stats