Place this clip at the reveal of the wrong answer in your edit, then follow it with the shocked sound for the reaction shot and the oh no no no laugh as the scene moves on to the next fail.
關於此聲音
This is a short voice clip of a kid being asked what 9 plus 10 is and confidently answering 21 instead of 19. The delivery is dead serious for such a wrong answer, which makes it feel funny, dumb and instantly quotable. It comes from the 9+10=21 meme exchange referenced in its title.