I have difficulty answering this question:

Suppose that in one class, the average gas purchase for my students was 7 gallons per week with a standard deviation of 2 gallons. Then, I can estimate that 68% of my students will purchase between 5 and 9 gallons, 95% will purchase between 3 and 11 gallons, and almost everyone in the classroom (99.7%) will purchase between 1 and 13 gallons each week. This is the picture to clarify the question.