There are two proposed systems for number processing: one that precisely represents small numerosities (<4), the Parallel Individuation (PI) system, and one that encodes approximate large quantities, the Approximate Number system (ANS). Research suggests that these systems contribute to math performance. Nevertheless, most studies have focused on one of the two systems, mostly on the ANS, and have only investigated them in childhood and adulthood. Adolescence is a critical period for developing the math skills required for adult life. We investigated how the ANS, and in particular the PI system, and their neural underpinnings relate to different levels of mathematical achievement during adolescence. We examined numerical comparison abilities of 42 adolescents (mean age=16.34 years) with a wide range of math performance levels (1st-99th percentile), ranging from mathematical learning difficulties to mathematical talent. Adolescents compared quantities in the range of the PI system and the ANS during fMRI. Results showed that adolescents with lower math ability were less accurate than their peers with higher math ability, especially when comparing small quantities. At the brain level, adolescents’ math performance was associated with differences in task-related activity in a region involved in small quantity comparisons (right lateral occipital cortex) and in regions involved in comparisons across both quantity ranges (e.g., left hippocampus). Furthermore, low math ability was negatively associated with greater connectivity between these regions. These results suggest that mechanisms related to the PI system and general mechanisms associated with retrieval-based strategies are associated with different levels of mathematical achievement during adolescence.