The study focuses on the development of mental calculation of elementary students who show difficulties in learning math. In total, 20 children in 8 classes were observed during their first year at school. The math education of five classes was based on a special approach called “Zahlenblickschulung”, whereas three classes experienced more regular lessons. The collected data allowed a development of a typology of flexibility in mental calculation. Additionally, it was possible to describe the development of each student. The data analysis shows that instruction with “Zahlenblickschulung” also supports less advanced students in developing flexibility in mental calculation. Another result indicates that the recognition of number patterns and numerical relationships is crucial for learning to calculate (beyond counting).