Probability using confidence interval (cust. satisfaction)

Hi all, I need help on a probability and distributions question please.

The manager of a store would like to estimate the mean customer satisfaction level (on a scale of 1 to 20) by using confidence interval method (mathematical). How many customers would have to be sampled if the store manager wishes to estimate the mean to within 0.2, with a probability of 0.9? Assuming that s=2.155

Thanks in advanced

Re: Probability using confidence interval

You should have a formula in your textbook for calculating the sample size appropriate for a normal distribution assumption, given a known standard deviation, margin of error and confidence level.