A design engineer wants to construct a sample mean chart for controlling the service life of a halogen headlamp his company producesHe knows from numerous previous samples that this service life is normally distributed with a mean of $500$ hours and a standard deviation of $20$ hoursOn three recent production batches, he tested service life on random samples of four headlamps $,$ with these results : Sample $123$ Service Life $($hours$)495505500325515505515470480470460$ B$.507$ and $483$ D$.523$ and $515460$ and $440552$ and $544$ What are the x$-$bar chart upper and lower control limits $?$ and $475$