How do u do cross multiplication

Sales personnel for skillings distributors submit weekly reports listing the customer contacts made during the week. a sample of 55 weekly reports showed a sample mean of 17.5 customer contacts per week. the sample standard deviation was 5.8. provide 90% and 95% confidence intervals for the population mean number of weekly customer contacts for the sales personnel. a. 90% confidence interval, to 2 decimals b. 95% confidence interval, to 2 decimals