@Sophie_Libkind to comment 12:
I find it also a bit unclear what John means with "you have reached all its predecessors" By that he probably doesn't mean "had been at all predecessors states" but more "performed all predecessors tasks", because (at least for a production process) one usually doesnt stop in the predecessor states that is you usually don't leave things in an unfinished state, i.e. you usually need to do all tasks - unless some steps are redundancy steps. In the former interpretation 6 month would be enough since then state 50 had been reached through 10->30->50 and all predecessor states (which take at most 4 months) had been reached as well and there would be no slack time, but in the latter interpretation (the one I think is meant) one needs 7 months and E would have a slack time of 1 month. So it depends how much redundancy you want to/ are allowed to have in your system.