We report the results from a case study of Ariel, a middle-school participant in a 3-yr longitudinal study of the development of understanding of mathematical ideas. We focus on Ariel’s use of arithmetic knowledge in finding a rule for his solution to a problem task that takes the form of a composite function. Over a year later, after being introduced to the technical algebra language and formal notation, Ariel revisits the problem and offers a general, closed-form solution. When re-solving the problem his language is more precise as he connects meaning to the symbols. Viewing a video of his earlier solution, Ariel acknowledges the correctness of his earlier work and indicates that the earlier solution was not generalizable.