Solving exercises and Science problems promote the development of abilities related with the elaboration of new operating schemes as the consolidation of structures that have been built along the school work of the student. One of the aspects to be considered in the development of structures is that one referred to the conceptualization of the measurement and units system. Through the analysis of some problematic situations faced by High School students of first year (around 15 years old), higher level, we observe that there may be concepts in action and theorems in action, with their respective rules of action mostly related with those basic structures built in the mathematics field and with those strategies activated in daily life. The discrimination of the different type of decisions taken by the student while solving a problematic situation, carried out on the light of the Theory of Conceptual Fields of Vergnaud, invites us to think over the differences in the level of difficulty present in every magnitude as well as the need of making a critical review of the equivalence concept.