The continuing difference between times on problems where b is large and ones where it is small suggests that neither 6- nor 9-year-olds have sufficient conceptual competence (i. e., understanding of the inversion principle) to consistently answer such problems without adding and subtracting. Not until age 11 do most children, like almost all adults, ignore the particular value of b and solve all problems equally quickly, thus demonstrating understanding of the inversion principle.