A design engineer wants to construct SPC chart for controlling the service life of a halogen headlamp his company produces. On three recent production batches, he tested service life on random samples of four headlamps, with these results:

$\backslash$table$[[$Sample$,$Service Life $($hours$)],[1,495,500,505,500],[2,525,515,505,515],[3,470,480,460,470]]$

If it is know that the process standard deviation is $20,$ and the exfacted tolerance limits are $420,580,$ what is the process capability index from the sample?