Problem: An online store wants to estimate the intervals on the time customers spend on the website before purchasing an item. The population mean of a set of data is unknown. The sample mean...

Problem:

An online store wants to estimate the intervals on the time customers spend on the website before purchasing an item. The population mean of a set of data is unknown. The sample mean is 18 minutes, and the error bound for the mean is 10, at a 95% confidence level. (So, x¯=18 and EBM=10.)

Questions:

According to my text book, the standard deviation to the problem above is 5.

Please explain the calculation on how to get the standard deviation of 5 for this problem. Thank you!