This paper studies the difficulties children have solving word problems that correspond to one of the situations modeled by multiplication; that of a ratio (e.g. "candies per box"), multiplied by an extensive quantity (e.g. "boxes") (I x E). Sixty children were individually observed solving and repeating I x E problems. Various tasks designed for this study were used to probe children's representation of the problem: drawings, model construction, and model recognition. Systematic errors in children's performance on these tasks were analyzed. The results of children's performance show that the major source of difficulty in solving I x E problems lies in a component of this process not common to all multiplication problems, understanding the concept of a ratio. This conclusion underscores the importance of conceptual understanding in solving word problems. The relevance of the study's results for teaching and research of word problems is discussed. (Author)