Data was collected for 290 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.

20 40 60 80 100 120 140 160 180 200 Number of People in 10 min Frequency [Graphs generated by this script: setBorder(54,40,20,5); initPicture(82,200,0,91);axes(1000,20,1,1000,20); fill="blue"; stroke="black";line([100,-1.82],[100,1.82]); text([100,0],"100","below");line([120,-1.82],[120,1.82]); text([120,0],"120","below");line([140,-1.82],[140,1.82]); text([140,0],"140","below");line([160,-1.82],[160,1.82]); text([160,0],"160","below");line([180,-1.82],[180,1.82]); text([180,0],"180","below");line([200,-1.82],[200,1.82]); text([200,0],"200","below");textabs([165,0],"Number of People in 10 min","above");textabs([0,115],"Frequency","right",90);rect([100,0],[120,90]);rect([120,0],[140,25]);rect([140,0],[160,67]);rect([160,0],[180,91]);rect([180,0],[200,17]);]

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.