On average, how much time does a customer spend in the system in a single$-$server model? $($minutes$)$

$12\mathrm{.}71$

$10\mathrm{.}25$

$9\mathrm{.}52$

$1\mathrm{.}95$

$21\mathrm{.}5$

$4\mathrm{.}83$

$8\mathrm{.}41$