The answer was 15, but since the lecturer made an error, the "500 server error" error appeared, making the dude that was laughed at, for saying 500, correct.
Sorry if my explanation is too confusing.
Fairy Tail during the Grand Magic Games arc.
I'm not sure about the episode, but it's probably 175 or 176.
It's called "The Dragon Graveyard" if I'm not mistaken.
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