Data was collected for 362 randomly selected 10 minute intervals. For each ten-minute interval, the number of people entering the atrium of a large mall were recorded. The data is summarized in the histogram below.

[Graphs generated by this script: setBorder(54,40,20,5); initPicture(42,160,0,95);axes(1000,20,1,1000,20); fill="blue"; stroke="black";line([60,-1.9],[60,1.9]); text([60,0],"60","below");line([80,-1.9],[80,1.9]); text([80,0],"80","below");line([100,-1.9],[100,1.9]); text([100,0],"100","below");line([120,-1.9],[120,1.9]); text([120,0],"120","below");line([140,-1.9],[140,1.9]); text([140,0],"140","below");line([160,-1.9],[160,1.9]); text([160,0],"160","below");textabs([165,0],"Number of People in 10 min","above");textabs([0,115],"Frequency","right",90);rect([60,0],[80,71]);rect([80,0],[100,77]);rect([100,0],[120,95]);rect([120,0],[140,47]);rect([140,0],[160,72]);]

What is the class width for this histogram (and corresponding GFDT)? Note: Each class contains its lower class limit, but not its upper class limit.

Class width =