Suppose we want a 90% confidence interval for the average amount of time (in minutes) spent per week on homework by the students in a large introductory statistics course. The interval is to have a margin of error of 2 minutes, and the amount spent has a normal distribution with a standard deviation = 30 minutes.

My answer was based on the formula estimate+/- margin of error, but I had it wrong. Would you mind delivering your answer with a a detailed explanation so I know how to solve these problems?