The Range Rule of Thumb says that the range is about four times the standard deviation. (i.e. two standard deviations to the left and two standard deviations to the right of the mean).
Given that the mean is 500 and the standard deviation is 50, then
The minimum and the maximum "usual" values are given by
[tex]500\pm2(50)=500\pm100 \\ \\ =(500-100,\ 500+100)=(400,\ 600)[/tex]
Therefore, the minimun "usual" value is 400 while the maximum "usual" value is 600.
