These two statements are contradictory. 40 hours over the course of a summer is not “immense.” When I was in HS, I did some volunteering over the summer, in addition to a job that paid. The cost for me was about 4 euro 50 a day for public transportation, which is far less than the $5K some of these outfits charge for the “privilege” of travelling overseas.
Saying that one is a “published author at 17/18” by itself is less-than-impressive, IMO. It’s possible to write a “book” over a weekend and get it self-published and make that claim. Now being a published author of a book that is a best-seller and/or is critically acclaimed would be something to mention.
Also, I’m not a native-English speaker, so I could be wrong, but I believe that it’s “amount of time” or “number of hours.” Not a big deal except for a published author looking to study English at Yale.