Rosal Monserrat can cast Prestidigitation, Light, Detect Magic, Comprehend Languages, and Endure Elements. That is how outstanding her mathematical talents are, how good with numbers she was as a child and delighted in them; that as a successful merchant's sixth daughter, sired on his third wife, she was given that much of a wizarding education. Not taught in a wizard academy, of course, but permitted to hang about the magic lessons in which his third son was being tutored.
A year or so after those lessons ended, the house's fifth daughter was due to be married off to an older man, rich and successful, strong-looking and with the appearance of a short temper; to Rosal, he seemed rather like the sort who might hit her sister. Surely, she had thought, a Good person would do something about that - though she obviously had little to no power, in her household, and the alliance was not one her father could afford to lose -
Rosal Monserrat, who was not as pretty but did have that wizardry going for her, put forth her best effort at seduction and managed to get herself inserted in her sister's stead.
It is a year later now, and Rosal Monserrat is reading in her bedroom by the glow of her Light spell, feeling miserable, with fresh bruises yellowing about her face, her eyes, her thighs.