Yeah, I interpreted "where N is the Return Trip" as a way of creating a phrase meaning 'Return Trip' and then substituting that into the first part of the statement. I think this is what Zarban intended, but, for some reason, I was expecting the "where N is..." portion of the title to still persist, along with the substitution that was made into the earlier part. In other words, I wasn't expecting the "where N is..." portion to disappear from the final result, but rather to remain somehow a part of it, and it was this double-presence that stymied me. I don't know why I thought this, though.

I'm not faulting Zarban; he's trying to find interesting ways to convey information, and it's up to me to keep up. It's a bit like learning the foibles of a cryptic crossword puzzle compiler, really.