Studies have found that successful problem solving with 2-digit numbers depends on children’s ability to construct a concept of ten that is both a collection of ones and a single unit of ten that can be counted, decomposed, traded and exchanged for units of different value (e.g., Cobb & Wheatley, 1988; Fuson et al., 1997; Ross, 1989; Steffe, Cobb & von Glasersfeld, 1988)