In Hogwarts Legacy, the Arithmancy Classroom is in the upper level of the Library Annex, and is itself locked behind a puzzle door. Once you get inside the Arithmancy Classroom, you’ll discover two more puzzle doors side-by-side, so it’s time to practice some more arithmancy (i.e. math magic) in order to open them and get some loot.
Related: How to complete the Hall of Herodiana in Hogwarts Legacy
How to open the doors in the Arithmancy Classroom in the Library Annex
In order to have gotten into the Arithmancy Classroom in the first place, you will have had to solve the Divination Classroom door puzzle, so you know how these puzzles work. The creature icons around the door correspond, left to right, to the numbers 0 to 9, and you have to roll the triangles at the side so that the numbers of the triangle points on the doors add up to the numbers inside the triangles on the doors.
The top triangle on the left-hand door reads quintaped (5) + salamander (6) + ? = 15, so the missing creature icon must be owl (4), so roll the triangle to the left of the door until it displays an owl. The bottom triangle reads 13 + demiguise (0) + ?? = 18, so the missing icon must be quintaped (5), so roll the triangle to the right of the door until it displays a quintaped. Now you can open the door and loot the study, where you’ll find two chests, one of which contains the Trees Conjuration exploration collectible.
The top triangle on the right-hand door in the Arithmancy Classroom reads 6 + quintaped (5) + ? = 17, so the missing icon must be salamander (6), so roll the triangle to the left of the door until it displays a salamander. The bottom triangle reads squid (7) + 0 + ?? = 8, so the missing icon must be unicorn (1), so roll the triangle to the right of the door until it displays a unicorn. You can now open the door and search the chests for loot, including the Medium Ground Coverings Conjuration exploration collectible.
