# Math Help - Confidence Interval

1. ## Confidence Interval

The city of Paducah wants to estimate the mean weight of plastic discarded by households in one week. How many households should be randomly selected if they want to be 99% confident that the sample mean is within 0.250 lb. of the true population mean. A previous study yielded a standard deviation of 1.065 lb.

I didn't really understand the replies to the last questions i posted. Maybe someone can help with this one?

2. Originally Posted by scoober
The city of Paducah wants to estimate the mean weight of plastic discarded by households in one week. How many households should be randomly selected if they want to be 99% confident that the sample mean is within 0.250 lb. of the true population mean. A previous study yielded a standard deviation of 1.065 lb.

I didn't really understand the replies to the last questions i posted. Maybe someone can help with this one?

you set $z_{\alpha/2}\sigma/\sqrt{n}\le .25$ and solve for n.

$\sigma=1.065$ and $z_{.005}= 2.576$, via the last row of a t-table.