Fifty years after the birth of duty-free shopping at international airports and border-crossing facilities, the European commission announced plans to end this form of business. A study by Cranfield University was carried out to estimate the average percentage rise in airline landing charges that would result as airlines try to make up for the loss of on-board duty-free shopping revenues. The study found the average increase to be 60%. If this was based on a random sample of 22 international flights and the standard deviation of increase was 25%, give a 90% confidence interval for the average increase.