Proportional reasoning failures seem to constitute most errors in probabilistic reasoning, yet there is little empirical evidence about its role for attaining probabilistic knowledge and how to effectively intervene with students who have less proportional reasoning skills. We examined the contributions of students' proportional reasoning skill and example-based practice when learning about probabilities from a reformed seventh grade curriculum. Teachers in their regular classrooms were randomly assigned to instruct with a reformed textbook (control) or a version revised to incorporate correct and incorrect example problems with prompts to explain (treatment). Students' prior knowledge in proportional reasoning skill separately predicted probabilistic knowledge at posttest, regardless of their prior knowledge in probability or minority status. Overall, students in the treatment condition improved more in their probabilistic knowledge, if they started with less proportional reasoning skills. Our findings suggest that example-based practice is beneficial for students with less prior knowledge of proportions, likely a key concept for developing probabilistic knowledge.