An assembly line has $5$ workstations: A$,$ B$,$ C$,$ D and E which are arranged as a flowline. Each workstation performs one task. The processing time of each task $($task $1,2,3,4,$ and $5)$ performed in the respective workstation $($A$,$ B$,$ C$,$ D and E$)$ is $5$ min, $3$min, $6$min, $4$min, $2$min, respectively. Tasks are performed sequentially as in the order of task $1,2,3,4,$ and $5,$ in workstation, A$,$ B$,$ C$,$ D$,$ and E$,$ respectively. The efficiency is: