Abstract
Subtraction has two meanings and each meaning leads to the different strategies. The meaning of “taking away something†suggests a direct subtraction, while the meaning of “determining the difference between two numbers†is more likely to be modeled as indirect addition. Many prior researches found that the second meaning and second strategy rarely appeared in the mathematical textbooks and teacher explanations, including in Indonesia. Therefore, this study was conducted to contribute to the development of a local instruction theory for subtraction by designing instructional activities that can facilitate first grade of primary school students to develop a model in solving two digit numbers subtraction. Consequently, design research was chosen as an appropriate approach for achieving the research aim and Realistic Mathematics Education (RME) was used as a guide to design the lesson. This study involved 6 students in the pilot experiment, 31 students in the teaching experiment, and a first grade teacher of SDN 179 Palembang. The result of this study shows that the beads string could bridge students from the contextual problems (taking ginger candies and making grains bracelets) to the use of the empty number line. It also shows that the empty number line could promote students to use different strategies (direct subtraction, indirect addition, and indirect subtraction) in solving subtraction problems. Based on these findings, it is recommended to apply RME in the teaching learning process to make it more meaningful for students.
Highlights
In solving subtraction problems, students have to think about the meaning of subtraction and the more efficient strategies to solve it
After finding the difficulty in counting, the students would realize to switch their strategy into indirect addition or indirect subtraction
The beads string helped students as a “model of” the situation in solving subtraction problems and it functioned as a bridge in moving from the contextual problems to the use of the empty number line
Summary
Students have to think about the meaning of subtraction and the more efficient strategies to solve it. According to Fosnot and Dolk (2001), subtraction has two meanings; those are “taking away something” and “determining the difference between two numbers”. The limited research interest for this complement strategy of direct subtraction is quite surprising because there are indications that indirect addition is computationally remarkably efficient and very promising from a broader educational perspective (Torbeyns et al, 2009). In a traditional teaching learning method, teachers teach students an algorithm of subtraction directly, subtracting tens and ones separately, after they learn subtraction up to 20 by doing physical activities or using drawing. It is meaningless for students because they do this procedure without understanding (Kamii & Lewis, 1993)
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