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Welcome to the CPL Performance question and answer forum. Please feel free to post your questions but more importantly also suggest answers for your forum colleagues. Bob himself or one of the other tutors will get to your question as soon as we can.
In the case of this question, the cruise fuel consumption happens to be 60 litres/hr. That's one litre per minute. Since the trip fuel is 64 iitres, the safe endurance is 64 minutes (64 litres @ 60 litres/hour). The formula says Time to PNR = 64 x 190 ÷ 340. The 64 is minutes, it just happens also be 64 litres in this case.