Hi RedonYellow, your links to these questions are broken.
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
And this one with a t one