I don’t think any (reasonable) teacher would care either way, letting n represent the number of passengers, or letting n represent the number of empty seats. The important thing is communication: Simply state up front what your “n” is representing.
And oh yeah, that sure seems like a stupid-ass way to rent a plane. 
That IS the best part. 
So many algebra problems hinge on defining one term as x and one as (total - x). I once caught a textbook author defining them incorrectly in an example problem when I was proofreading his book. I wrote a lengthy but tactful query, and he thanked me for catching the goof.
So yeah, it can be confusing! :smack: