7.78 Wosiwosi, a UK-based African food store, specializes in the distribution of wholesale and retail of African and Caribbean food. Suppose the manager wants to estimate the mean amount customers will spend on their next visit. An analyst studies customers’ expenditures per visit. From a random sample of 20 shoppers, she records their total expenditure, rounded to the nearest pound, as follows:

265 256 164 257 158 153 214 265 171 162 234 279 151 277 245 279 166 194 214 161

a. Find a 90% confidence interval for the average expenditure amount of all customers who visited the Wosiwosi store. State the assumptions.

b. Calculate the widths for 95% and 99% confidence intervals.