# Probability of balls chosen from an urn

1. Sep 25, 2007

### Gott_ist_tot

1. The problem statement, all variables and given/known data
An urn contains 3 white and 4 red balls. A ball is chosen at random ( in a frequency sense) and replaced. What is the probability of the first 5 balls chosen exactly 2 will be white.

2. Relevant equations

3. The attempt at a solution

Really I haven't made much progress. I do not understand the significance of the balls being replaced. If they weren't replaced that would make sense to me. But with them being replaced I have no idea. Thanks for any help and advice.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Sep 25, 2007

### nicksauce

Because they are being replaced, the chance of getting a white ball will always be 3/7.

So it's like, what is the probability that if you flip a coin 5 times you will get exactly two heads (only instead of 50/50, then chance of getting heads in 3/7).

3. Sep 25, 2007

### Dick

The significance of the balls being replaced just means every time you draw a ball you have a 3/7 chance of it's being white and 4/7 of it's being red. So what are the odds of getting 2 white and 3 red. Binomial distribution. Go for it Nietzsche!

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