- A one-way journey takes exactly 7 x 24 hours.
- The ships leave at noon, local time.
- The ships leave at the first noon that takes place after arriving.
Noon in New York happens 6 hours after noon in Le Havre. This means that 6 hours after a ship leaves Le Havre, a ship sets out from New York. Also, just as a ship leaves Le Havre, a ship is arriving in New York. At 11:55 our ship is waiting to depart. At the same time, 7 ships are west-bound, in transit to New York. One is 5 minutes away from arriving in New York. The others are 1, 2, 3, 4, 5 and 6 days — plus 5 minutes — from arriving in New York. Also, in the east-bound lane, another 7 ships are heading for Le Havre. One is 6 hours from arriving. The others are 1, 2, 3, 4, 5 and 6 days — plus 6 hours — from arriving in Le Havre. Our ship must meet all of these. That makes 7 + 7 = 14 ships.
EDIT: The reason for these assumptions is that without them, we have a wide host of ambiguities that need to be resolved.
When does the New York ship leave? Is it noon New York time or noon Le Havre time (i.e. 0600 in the morning in New York)?
What does it mean that a passing takes "7 days and 7 nights"? Is it 7 x 24 hours or does it mean that the journey always ends at noon, local time?
Do the ships idle in port or not? Do they stay for unloading and reloading, or do they depart instantly?
What does "meet during the passing" mean? If the ship can be in two states — Passing and Idle — when does it move from one state to the other? Because this would mean that a ship that departs a port at noon while another arrives at the same time do not meet during the passing, because they go from Idle - Passing to Passing - Idle at the same instant... they are never Passing-Passing at the same time. Or are they?! Is it perhaps three states: Idle, berthing/unberthing (does this count as "during the passing" or not), and Passing? And if that is the case... when does berthing/unberthing start and end?!
All of these ambiguities are resolved with the assumptions I make, and it even makes sense from a realistic point of view...
A ship that arrives in New York has from 0600 to 1200 to unload/disembark and reload/embark. A ship that arrives in Le Havre at 1800 in the evening has time to let the stevedores go home for a good night's sleep, then come back at 0600 in the morning and start unloading/reloading the ship in the same time that the New York stevedores have. Also it makes sense that travelling the same distance takes the same amount of time. And finally it makes the schedule make sense: "The service departs every day at noon. No no good Sir/Madam, don't bother with time zones, it's noon local time. Easy to remember, just look at the nearest public clock and you will know").
For the record I'm a software developer and these kinds of ambiguities is what makes reading requirements specifications so much "fun"...