You put it very well Table; the winning answer and the right answer are two different things.
To calculate the distance, you need the 10 hrs and 10 mins travel time the Gauls have. Coz when the prefect tells everyone the Gauls will be attacking in 10 hrs, the Gauls are already en route for 10 mins. This info is vital.
To calculate the travel time for the Romans, you need to start calculating from the time the prefect talks, which leaves you 10 hrs. Then the Romans have 2 hrs and 30 mins preparation time. You have to add that to the actual travel time for the Romans.
The 55 minutes that are now being awarded imply that the Romans need to hit that Gaul village within 10 hrs and 10 mins. This isn’t right; they need to hit it within 10 hrs. Bottom line: the Romans will hit the Gaul village 45 mins before the Gauls are supposed to hit the Roman village.
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The above was regarding the travel time.
About being able to destroy the Gaul village... The Romans are on their way with catapults... no rams. What if that Gaul hammer village has a lvl 20 wall (one might assume it will be there)...? Then it might well be that the Romans won't be able to destroy that Gaul village.