What is the ruling on a female mathematics teacher teaching mathematics, and is her salary considered unlawful if this subject is not beneficial for women, as mentioned in one of the fatwas?

The fatwa mentioned is incorrect. The correct view is that it is permissible for both men and women to learn mathematics, and they will even be rewarded for it if their intention is good. Sciences are divided into two categories: religious sciences, which are sought for the sake of Allah, and worldly sciences, for which there is no harm in seeking worldly benefits, though benefiting Muslims through them is better. Mathematics is not a useless science; rather, nations are built upon it, and it is essential for various chapters of Islamic jurisprudence, such as zakat and inheritance. Furthermore, it is impossible to prepare a force for jihad against the enemy without it, and it is considered a fard kifayah (collective obligation). Women need it for themselves and for educating their children; it also serves as a mental exercise for them. Whoever prohibits or prevents her from it has slandered. It must be taught in accordance with Islamic guidelines.