# Thread: red balls and blue balls

1. ## 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. ## Re: red balls and blue balls

Originally Posted by timlazisa
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?

3. ## 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. ## Re: red balls and blue balls

You know how to use the table of the standard normal distribution?

5. ## Re: red balls and blue balls

Hi

yes I have this

6. ## Re: red balls and blue balls

Originally Posted by timlazisa
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 is . So . Denoting by Z the standard normal random variable
Now just look the value in the standard normal table.

7. ## 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. ## Re: red balls and blue balls

Originally Posted by timlazisa
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.

9. ## another one

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

10. ## Re: red balls and blue balls

I think this is a typical exercise of conditional probability.

If E and F are two events, so

since

And remember the formula of Bayes
If E is an event, so

If you do not understand I recommend you take a look at "A first course in probability, Sheldon Ross" in Chapter 3

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