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Flexible thinking ability of student to determine student who have flexible thinking ability can be seen fro, several aspect are flexible, fast and varied. In solving mathematical problems each student have different characteristics. The researcher knows that differences from the student’s ability to solve mathematical problems. The purpose of this research to know flexible thinking ability of student in solving open ended. Subject this research are student in MTs N 1 Semarang grade VIII. Research’s instrument that used are test questions in mathematical ability for determine the subject, test question in solving open ended mathematical problems to know flexible thinking ability of students and interview guidelines. The research was conducted in testing mathematical ability of 27 students to determine three(3) categories student 1 which are student who have high math ability, 1 which are student who have middle math ability and 1 which are students who have low math ability. After that the researcher do open ended mathematical problem solving and interview to each subject. The result of test and interview are 1) subject Subject who have high math ability they are flexibly to understood and plan problem solving, the subject fast to solve the problem and give several alternative answe 2) Subject who have middle math ability they are flexibly to understood and plan problem solving, the subject fast to solve the problem and give several alternative answer 3)subject who have low math ability they are less flexibly to understood the problem plan problem solving can quickly solve the problem and give several alternative answer, but less precise.
Keyword : flexible thinking, solving open ended mathematic problems, student math ability.