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 this service life is normally distributed with a mean of 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 $2$ Service Life hours $495500505500525515505515470480460470500$ hours E none of the given choices C$487$ hours A $250$ hours What is the value of the center line for the x$-$bar chart $?$ hours