"I know what one must do to be happy."
Translation:Je sais ce qu'il faut faire pour être heureux.
Marked down for 'content' which Duo has previously translated as 'happy'. Collins gives 'heureux' for 'happy' and defines 'content' as 'pleased, glad' without any overlap. I trust Collins more on this than Duo and have reported, suggesting they review how they translate 'content' passim.