Background: Math is a difficult subject with which many upper middle-school students struggle. In addition to knowledge, skills, and motivation, effective learning strategies play an important role in learning math. Aims: This study aimed to examine the effects of comprehension-oriented learning strategies (perceptual and abstract grouping of material) -- together with math self-concept, task-persistent learning behaviour, earlier math skills, reasoning abilities, and reading comprehension -- on solving mathematical calculation and word problems at the end of middle school. Sample: The sample consisted of 231 students (50% boys; mean age in Grade 9 = 15.55 years) from 16 schools and 31 classrooms in Estonia. Methods: Students were tested twice -- once at the end of Grade 6 and once at the end of Grade 9. Calculation and problem-solving skills were assessed at both time points. Math self-concept, teacher-rated task persistence, reasoning abilities, and reading comprehension were assessed in Grade 6. Learning strategies were assessed using a word-memorization task in Grade 9. Multiple regression analysis was used. Results and Conclusions: Applying abstract (but not perceptual) grouping was related to higher calculation and problem-solving skills. Math skills were persistent over time, and calculation and problem-solving skills mutually affected each other. Math self-concept, task persistence, and reading comprehension had a positive effect on problem-solving but not calculation skills. The study noted deficient use of comprehension-oriented strategies at the end of middle school, suggesting that more attention should be paid to teaching and discussing such learning strategies. These findings suggest that students' math skills may be enhanced by supporting metacognitive awareness and use of comprehension-oriented learning strategies.