For this question, I selected all the checkmarks. In that first period from 5:00pm - 5:12pm, 12 min pass by, but also in that same interval, it is true that 2 min passed, 3 min passed, … etc and so on. Why is this not the right answer? During that 12 min period, no trains departed. So it seems like an upper bound and everything <= 12 should be applicable right?
The wording on the question and the solution you are seeing is slightly - but crucially - different. Please recheck and let us know if you’re still seeing the ambiguity in the version given in the quiz.