As American consumers become more careful about the foods they eat, food processors try to stay competitive by avoiding excessive amounts of fat, cholesterol, and sodium in the food they sell. The following data are amounts of sodium per slice (in milligram) for each of the eight brands of American cheese.

263 288 307 320 326 340 345 520

What is the average amount of sodium per slice for the eight brands of cheese? Write the answer with one decimal place.