In 2020, two teachers and seven students participated in an action research project that sought to improve the teaching and learning of fractions in a New Zealand primary school. This research revealed that the collaborative creation of hypothetical learning trajectories was beneficial to the teachers' practice, content knowledge, and confidence, as well as the learning design and student achievement. The participating teachers employed pedagogical strategies through the hypothetical learning trajectories, which then trickled down into their learning design. It supported their students to move from procedural application to conceptual understanding.