A city planner wants to estimate the average monthly residential water usage in the city. He selected a random sample of 39 households from the city, which gave the mean water usage to be 3410.30 gallons over a one-month period. Based on earlier data, the population standard deviation of the monthly residential water usage in this city is 387.90 gallons. Make a 95 % confidence interval for the average monthly residential water usage for all households in this city.