Consider the following three-step assembly operation with quality problems. All steps are staffed by a dedicated employee (so that the different steps can be performed in parallel).

The first step has a processing time of 7 minutes per unit. The second step has a processing time of 6 minutes per unit.  The third step has a processing time of 5 minutes per unit. 

With a 40% probability, the units coming out of the third step have to be reworked. In that case, the operations at the second and third steps are repeated. 

You can assume that rework always succeeds (i.e. a unit going through the rework loop will always work after the third step) and the processing times for units in rework are the same as for regular units.

Which step is the bottleneck of the process?