$($b$)$ Another four$-$station line $($single machine in each station$)$ is being designed to

replace the line in Part $($a$).$ Assume that the machine in each station has the average

processing time of $3$ minutes, but the new balanced line is expected to run under the

Practical Worst Case $($PWC$).$ What is the minimum WIP level in order to keep up with

the same throughput as the old line in Part $($a$)?$