The results of the study are at variance with important aspects of models of children's performance proposed by Briars and Larkin and by Riley, Greeno, and Heller. A model of subtraction development and the computing difficulties and research issues suggested by the model are outlined. Counting down requires an ability to count backward while keeping track of the number of backward steps. Verbal solutions are not always parallel to object solutions. Two correct methods exist for counting down a certain number of words, and these methods may interfere with each other.
Special problems exist with subtraction on the number line.
And subtraction is not just take away but has multiple situational interpretations.