Undoubtedly, individual interviews with students significantly contributed to the analysis of their knowledge and identification of their obstacles in the learning process. Children develop their understanding by constructing relationships, and in order to understand they must speak something and be able to comprehend the relationships (Carpenter et.al., 1999, p.53). So, the „treatment process‟ as part of action research methodology impacted directly the improvement of the student‟s ability to understand multiplication and division as inverse concepts and to solve different problems.