Abstract: Based on the production system theory, an information processing taxonomy (IPT) model was postulated to extend the production system by including a second level of production of information. The aim of this paper is to report on evidence from children’s thinking to verify the features of the model particularly the extension of the production system. The results of investigation show that both the written and verbal statements of children working on a ratio and proportion problem support the mathematical thinking features spelt out in the IPT Model.