Background: National and international assessments show South Africa's underperformance in mathematics. As many learners already fall behind in the early grades, where foundational number sense should be established, addressing the challenges of number sense in the Foundation Phase (FP) is important. Research recognises the need to better develop pre-service primary teachers' (PSTs') mathematical content knowledge (CK) and pedagogical content knowledge (PCK). Understanding the importance that CK PSTs bring to their tertiary studies is needed. Aim: The study explores, what knowledge do third-year FP PSTs have of additive reasoning mental mathematics strategies? Setting: We share data from a larger study of PSTs' CK and PCK of additive reasoning strategies (a key aspect of number sense) before additive reasoning lectures with 80 third-year FP PSTs during their mathematics methodology course at a private institution. Methods: Participants included 54 of the 59 PSTs who agreed to take part in the study and were present in the first session when the pre-questionnaire (the focus of this article) was administered. Quantitative and qualitative analysis of PSTs' methods used, performance on the pre-questionnaire calculation items and explanations of methods and strategies were carried out. Results: Results highlight the need for explicit teaching of efficient calculation strategies along with required fluencies for using strategies. Conclusion: Knowledge of additive reasoning strategies cannot be taken for granted as established at school. Contribution: The findings of the study highlight the need for explicit teaching of efficient mental strategies such as Bridging Through Ten, Jump Strategy and Rounding and Adjusting in PSTs' studies.