How can I tell when to use the z-interval or t-interval

I know to use a t-interval when the standard deviation is unknown. However, the wording in these two problems is quite similar. The first problem is solved using a z-interval while the second is using the t-interval. I don't understand why? What should I look for here to determine whether to use a z or t interval?

The book solves this with a z interval

http://i233.photobucket.com/albums/e...isChan7/dm.jpg

And this one with a t one

http://i233.photobucket.com/albums/e...sChan7/dm4.jpg

Re: How can I tell when to use the z-interval or t-interval

Hi RedonYellow, your links to these questions are broken.

Re: How can I tell when to use the z-interval or t-interval

Sample size will dictate which to use. The t-dist has fatter tails with a lower peak. If the sample size is small, less than about 30, use the t-distribution.