# Thread: Urgent HW Project Due tomorrow =(

1. ## Urgent HW Project Due tomorrow Confidence Interval Problem=(

I'm stuck on this problem, i can't figure what the standard deviation is, and im not sure if 45 is the mean.

1) A new public telephone has been installed at the airport baggage-claim area. A quarter was lost 45 of the 300 times that it was used. Find a 95% confidence interval for the true proportion of times that a user will lose a quarter.

2. The proportion is approximatly normally distributed..
The sample proportion is $\displaystyle \bar{Y}=\frac{45}{300}=0.15=p$

So the 95% CI is given by
$\displaystyle \bar{Y} \pm z_{.05/2} \sqrt{\frac{p\cdot (1-p)}{n}}$

3. hey thanks rob youre the greatest ! rob is that the answer or a setup for the problem?

would n=300?

4. if the confidence level is 95%, then the z score would be 1.96?

5. Thats the setup for the answer. Yeah, n is equal to 300, so just plug the numbers in to get the lower and upper confidence intervals. And $\displaystyle z_{.05/2}=1.96$

6. thanks =)!