“…Children's strategy variety has been studied in diverse mathematical domains, including the domain of multi-digit addition and subtraction (Baroody 1987;Beishuizen 1993Beishuizen , 1999Blöte, Klein, & Beishuizen 2000;Blöte, Van der Burg, & Klein 2001;Carpenter, Franke, Jacobs, Fennema, & Empson 1997;Carroll 2000;Fuson 1992Fuson , 2003Fuson et al 1997;Heirdsfield & Cooper 2004;Hiebert & Wearne 1993, 1996Jones, Thornton, & Putt 1994;Klein, Beishuizen, & Treffers 1998;Murphy 2004;Thompson 1999Thompson , 2000Torbeyns, Verschaffel, & Ghesquière 2006). These studies first revealed that children generally apply three types of strategies to mentally solve sums and differences in the number domain 20-100, namely decomposition, sequential, and shortcut strategies.…”