Data was collected for 389 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.

20406080405060708090Number of People in 10 minFrequency[Graphs generated by this script: setBorder(54,40,20,5); initPicture(31,90,0,98);axes(1000,20,1,1000,20); fill="blue"; stroke="black";line([40,-1.96],[40,1.96]); text([40,0],"40","below");line([50,-1.96],[50,1.96]); text([50,0],"50","below");line([60,-1.96],[60,1.96]); text([60,0],"60","below");line([70,-1.96],[70,1.96]); text([70,0],"70","below");line([80,-1.96],[80,1.96]); text([80,0],"80","below");line([90,-1.96],[90,1.96]); text([90,0],"90","below");textabs([165,0],"Number of People in 10 min","above");textabs([0,115],"Frequency","right",90);rect([40,0],[50,91]);rect([50,0],[60,98]);rect([60,0],[70,90]);rect([70,0],[80,18]);rect([80,0],[90,92]);]

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 =