IUPAC Name clarification
The one with the lowest set of locants is correct. I.e. the one with lower first number in the locant set; if they are same, then the second one, etc. But, it is done on the ordered sets of locants, (3,6) vs (2,5) not (5,2). The substituent prefixes are then sorted alphabetically. So, the correct name is 5-ethyl-2-fluoroheptane.