Learning emphasizing fractions as a part-whole concept causes several limitations in developing fraction knowledge and inhibits proportional reasoning. We use fractions as quotients as the first context introduced in our learning trajectory. We report the teaching experiment results using the improved learning trajectory on thirty 4th grade students in Jakarta's public schools. The findings of this study indicate that the fractions as quotients used as the first stage in the learning trajectory can lay a solid foundation for the concept of partitioning in a variety of strategies and the concept of fractional parts. Besides, the developed learning trajectory has provided opportunities for students to learn about fractional mental operations, which are interrelated and serve as the basis for the development of proportional reasoning.