According to cognitive load theory, an expert learner can access prior knowledge to interpret a diagram with minimum effort. A novice learner who lacks prior knowledge, in contrast, would experience a high level of cognitive load when processing a diagram, which would interfere with learning. On learning to solve percentage problems, we investigated the effect of approach (non-algebra vs. algebra), with and without diagram support for two levels of learner expertise. Participants were 218 Asian students whose mean age was 15.00 (SD = 0.18). Contrary to our hypothesis, the presence or absence of a diagram may bore little, if any, benefit for students who were more knowledgeable. Considering two levels of learner expertise together, as hypothesized, performance outcomes favoring more knowledgeable students were greater for the unitary-pictorial approach than the equation-pictorial approach. Interestingly, the impact of a diagram was more pronounced for more knowledgeable students; however, with relevant prior algebra knowledge, the presence of a diagram (e.g., equation-pictorial approach) can also be helpful for less knowledgeable students. Overall, then, our research undertaking has yielded important implications for teaching and research purposes.