The purpose of this study was to examine the effects of a Tier 2 supplemental intervention focused on rational number equivalency concepts and applications on the mathematics performance of third-grade students with and without mathematics difficulties. The researcher used a pretest-posttest control group design and random assignment of 19 participants to core instruction and 19 participants to core + intervention instruction. Intervention instruction included the ratio interpretation of rational number in conjunction with explicit and conceptual instructional components--judicious review opportunities, concrete and visual representations, systematic task progression focused on big ideas, teacher and student verbalizations, and multiple opportunities for practice--to deliver a sequence of 20 intervention sessions over a period of 4 weeks. Students receiving core instruction plus intervention outperformed those receiving only core instruction on a standard measure of the intervention content as well as a pretest-posttest measure reflective of the core curriculum.