The purpose of this study is to review the number groups preferred by primary school 2nd grade students when solving flexible problems that require addition and subtraction with natural numbers, the reasons for choosing such groups and the strategies they develop when adding and subtracting. This study has a basic qualitative study pattern. Study participants consisted of 56 primary school 2nd grade students. Three different flexible problem forms, each one consisting of two questions, have been developed for the study to collect data. The two questions included in the problem forms are flexible problem expressions, one based on addition, the other based on subtraction. With flexible problem approach, students have been given the chance to pose and solve a problem by using a number group selected by themselves from the four different number groups provided to them. Other data obtained from the study have been subjected to descriptive analysis. According to study findings, the most frequent number pair preferred by students in solving addition and subtraction based flexible problems was “10 and multiples of 10”; while the lowest one was “numbers smaller than 10” in addition, and “numbers smaller than 20” in subtraction. Furthermore, the most popular strategy preferred by the students was “adding/subtracting unit digits first and then tens digits”, while the least popular was “rounding to 10” in addition, and “counting backwards” in subtraction. Such studies provide clues to lecturers about the sense of numbers of children.