another question:

our convenience store in Zap, ND. Let s assume your friend says that the average sale of all her customers is $23.00. Your friend does not know the standard deviation of the population of sales. (I guess that is why she hired you as her consultant.) You setup a random sampling technique whereby you ask your friend to randomly sample 50 customers in a particular month. Your friend reports that her sample of 50 customers had an average purchase of $25.00. Is this reasonable? In other words, what is the likelihood of this occurring? You will need to know the standard deviation of the sample... assume you calculated it and it was $3.00. Your friend wants to know if this was a true (reasonable) sampling of her customers. What do you tell her? Can you deduce if her business is increasing?

( now in this question what confidence interval do i have to use? my standard error when I used 80% confidence level is 42%. Can u help me with the steps of this question?

first I have to calculate upper and lower z score, then upper and lower bound?