Following Is Only the Start of a Child's Work (In

Essay

Following is only the start of a child's work (in base ten). What seems lacking in this child's understanding? (Assume, perhaps unfairly, that the child is not thinking of negative numbers.) $\begin{array}{c}\begin{array}{r}402 \\\underline{-39} \\\ldots 7 \end{array}\begin{array}{r}506 \\\underline{-149} \\ \ldots 3 \end{array}\end{array}$