constructions that may provoke the reversal errors of the "students-professors problem. " MacGregor & Stacey (1993) show that children do not follow a simple word-matching order and postulate the existence of mental models that can be accessed in any order. We hypothesise that the children's responses are a natural consequence of their previous development and show that, as the verbal problems become more complex, children are more likely to revert to a process-orientation with the arithmetic operation on the left, rather than an assignment order with the variable on the left. We place this in a theory of cognitive compression as the children grow in mathematical sophistication.