In recent studies, test-score increases have been shown in rule-based intelligence test tasks, such as number series, after watching a corresponding video tutorial. An open question remains regarding the mechanisms involved. Specifically, taking notes to describe the relations between numbers might be linked to test scores, and is hypothesized to mediate the effect of a number series video tutorial on number series test scores. Therefore, an experimental group (EG) watching a number series video tutorial (n = 58) was compared with a control group (CG) watching an irrelevant tutorial (n = 52) before working on number series items. Results revealed higher number series scores in the EG than the CG (d = 0.48), more items with provided notes in the EG than in the CG (d = 0.41), and substantial correlations between the number of items with notes and the number series sum scores in both groups (EG: r = 0.66; CG: r = 0.75). The effect of the video tutorial on the number series sum score was mediated by the number of items with notes (indirect effect = 3.41, SE = 1.74). Theoretical and practical implications as well as future research directions are discussed.