“…Answer: 2", leads to a faster and less error-prone answer than the direct subtraction strategy "81 -70 = 11 and 11 -9 = 2" (De Smedt, Torbeyns, Stassens, Ghesquière, & Verschaffel, 2010;Peters, De Smedt, Torbeyns, Ghesquière, & Verschaffel, 2012;Torbeyns, De Smedt, Ghesquière, & Verschaffel, 2009b;Verschaffel et al, 2007;Woods, Resnick, & Groen, 1975). The meaningful and flexible use of this strategy requires a good understanding of the numerical magnitudes in the problem and their mutual relation (Baroody, Torbeyns, & Verschaffel, 2009;Verschaffel, Bryant, & Torbeyns, 2012), and, more particularly, an appropriate estimation of the difference between minuend and subtrahend.…”