But when considering the detour (for purposes of a.) the restriction that the elves don't visit the same PO twice still applies, correct?
"Every time one of them reaches a post office, he either picks up all of the letters there or leaves all of them in the office."
That means that if one of the elves reaches a PO that has more letters than the remaining space in their backpacks they cannot pick up any letters, right? (Or can they take as many letters as they have space in their backpack?)
Are all the statements a.-e. referring only to the optimal solution found by Petra?
For example, when considering whether statements a. and e. are true (and some other one):
- a.: Steffan can modify Petra's solution and visit PO3 and still be back before Ralph
- e.: In Petra's solution Steffan returns exactly half an hour before Ralph
As opposed to
- a. + e.: Steffan can modify Petra's solution and visit PO3 and return exactly half an hour before Ralph
"At the end, I will tell you how many of you have guessed their hat colour correctly."
After Albert (and every subsequent pixie) makes their guess no-one gets any indication whether they were correct. It is only after they all guessed that they learn whether they were correct - right?