Their solution: Replace the (+) with a (z).
Via Newsweek:
Two years after the Islamic State militant group (ISIS) seized Raqqa and a number of areas in Syria, it successfully asserted itself and its way of thinking on most aspects of city life.
ISIS’s changes encompass the social, educational and cultural institutions. To make these changes, ISIS has at times used coercion and compulsion, and at other times enticed people through its interpretations of Islam.
The educational sector in ISIS territories is considered to have changed the most in comparison with educational curricula in other Syrian neighborhoods. The drastic change is the result of a series of decisions made by ISIS over the past two years, beginning with the creation of a bureau of education called the Diwan of Education; the name, and particularly the use of the word diwan, reflects ISIS’s attempt to redesign education in its captured territories based on its idea of Islam’s heyday.
The first of the diwan’s tasks was to dissolve the educational and administrative cadres in Raqqa’s schools and make them take mandatory courses on sharia. It called on them to repent and “purify their minds” to match its opinions on principles of nationalism, secularism and democracy, which it believes are contrary to Islamic sharia and must be corrected.
With the start of ISIS’s first school year in Raqqa in October 2013, committees affiliated with the militant group were established. The first educational curriculum covered primary school for children 7 to 11 years old. Pictures have surfaced of books containing the rules of Islamic manners, the fundamental concepts of tawhid (the fundamentals of Islamic doctrine) and the Arabic language. ISIS neglected subjects that had previously been taught, such as science, mathematics, art and music. […]
ISIS’s efforts at changing educational curricula have led it to adopt, at the beginning of the 2014 school year, new books for middle school children aged 12 to 15. The book Kitab al-Tawhid by the famous Islamic scholar Muhammad ibn Abd al-Wahhab—founder of Wahhabism—is one of the most important foundations of ISIS’s middle school education. Also used are materials such as the Prophet Muhammad’s hadith (sayings and traditions), Muhammad’s biography, Quranic sciences, Islamic jurisprudence and the Islamic doctrine.
Other traditional school subjects are incorporated at this level, including mathematics, science, physics, chemistry, the English language, composition, Arabic script, Arabic grammar, spelling, history, geography, vocabulary and certain literature.
ISIS has removed parts of Arabic literary history, keeping only the small part it considers “permissible.” ISIS considers other time periods in Arabic literature to be the literature of infidels.
Additionally ISIS changed the internationally acknowledged mathematical symbol for adding (+), replacing it with a new symbol represented by the letter z. ISIS’s reasoning is that the + sign indicates the cross, which is used worldwide as a symbol for Christians: using a plus sign would be imitating infidels and is thus forbidden.