A large, club$-$style grocery store is attempting to staff its check$-$out registers. Currently, the arrival rate is $\backslash$lambda $=135$ customers per hour and each server has an average service time of E$[$T$]=400$ seconds $($note this is roughly double the time for a regular grocery store$--$the higher time is to account for large, bulk items$).$ Our usual assumption holds where the interarrival and service times follow an exponential distribution.

Calculate the average service rate $\backslash$mu $($in hours to match $\backslash$lambda $).$