A design engineer wants to construct a sample mean chart for controlling the service life of a halogen headlamp his company produces. He knows from numerous previous samples that when this service life is in control it is normally distributed with a mean of $500$ hours and a standard deviation of $20$ hours. On three recent production batches, he tested service life on random samples of four headlamps, with these results:

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

What is the mean of the sample ranges, i$.$e$.,"$R$-$bar"?

$20$ hours

$10$ hours

$25$ hours

$5$ hours

$16\mathrm{.}6$ hours