Fiona Fang abacus mental arithmetic training class. Fiona Fang's abacus mental arithmetic training class in Nanning, Guangxi is the key specialty of Nanning's primary school mathematics training class. It is a well-known primary school mathematics training institution in Nanning. The addition and subtraction within the bishop's mental arithmetic of 20 is completely in line with the primary school mathematics syllabus. It is a simpler quick calculation subject than the primary school textbook, which simplifies the written calculation and strengthens the oral calculation.