Quantitative reasoning, although included in most science courses, can be challenging to teach. In this article, we explore whether cooperative learning may help instructors teach quantitative reasoning and enhance students' understanding and learning experience. Our lesson was taught in a large introductory geoscience course. The lesson required the undergraduate students to process geological data, represent the processed data graphically in a ternary diagram, and interpret the results in terms of geological environments. Students were assigned to groups in which they were asked to either work in pairs (experimental group) or individually (control group) on the tasks. Students' performance on questions related to ternary diagrams on the test and their feedback in the evaluation survey indicate that the cooperative approach enhances the ability of freshmen and sophomores to apply the quantitative reasoning they learned to new problems. Most participants prefer learning in a cooperative setting rather than the individual approach. We suggest that cooperative learning can help develop quantitative reasoning in undergraduate science classes.