Levels 21-30 are the Multithreading quest. You're in the Collections quest, which is the final quest of the course (i.e. levels 31-40). You can complete both final quests either consecutively or at the same time.
hi Alex, I actually switched to the Multithreading quest, and got the same "You've reached level 31" message. But as soon as one goes to the first lesson, it changes to the correct level, so it's just the welcome (landing page) message that is incorrect. No worries ;)
GO TO FULL VERSION