|"Harry, your eyesight really is awful."
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Sum-solving ink was a type of ink that presumably helped solve math problems. Luna Lovegood owned a bottle of turquoise sum-solving ink, but someone took it from her and hid it during the 1995-1996 school year, prompting her to post a notice asking for the return of it and several other missing possessions.
- Harry Potter and the Order of the Phoenix (film) (Mentioned on a poster)
- Harry Potter Film Wizardry (Mentioned on a poster)