Hi All,
On DS questions, when a prompt comes with a twist, the GMAT writers usually provide a "hint" about that twist in one or the other of the two Facts.
At first glance, you might not realize that the theater DID NOT NECESSARILY sell all of it's tickets for last night's performance. However, there ARE two clues that should have helped you to realize this possibility:
1) The prompt says that "...when all tickets are sold...the total revenue is $34,600", but then the question asks what the revenue was from last night's performance. There MUST be a twist of some kind, otherwise you would already have the answer to the question: $34,600 and the two Facts wouldn't need to be there.
2) Fact 2 tells us that while ALL of the balcony tickets were sold, ONLY 80% of the orchestra seats were sold. Here, you can catch the twist and go back to double check your work.
Since the other explanations in this thread explain the "math", I won't rehash any of that here, but it's worth remembering that DS questions are a test of a variety of your skills, including: organization, accuracy, thoroughness, attention-to-detail, pattern-matching, the ability to PROVE that you're correct....and some basic math skills. When you get a DS question wrong, the real reason probably won't be because of a math mistake. Get in the habit of taking LOTS of notes and doing organized work and you'll find most DS questions are pretty straight-forward.
GMAT assassins aren't born, they're made,
Rich