Interference in Solving Mathematical Problems Muhammad Irfan 1 , Cholis Sa’dijah 2 , Naufal Ishartono 3 , Sri Adi Widodo 4 , Arief Aulia Rahman 5 , Muhammad Nur Hudha 6 {muhammad.irfan@ustjogja.ac.id 1 , cholis.sadijah.fmipa@um.ac.id 2 , ni160@ums.ac.id 3 } Universitas Sarjanawiyata Tamansiswa 1,4 Universitas Negeri Malang 2,3 Universitas Muhammadiyah Surakarta 5 Universitas Kanjuruan Malang 6 Abstract. Many mathematical topics taught in schools have similarities in the structure of the problem, and the material is taught closely. Students often make mistakes in determining the problem solving method so that the answer is wrong. This error occurs because of the mixing of two knowledge that is close together so that someone is wrong in recalling the knowledge that should be used. In information processing theory, the event can be called interference. This paper will discuss how interference occurs in students' thinking processes when they solve mathematical problems. Subjects in this study were fourth semester students at one of the private universities in Yogyakarta selected purposively. This study uses an explorative qualitative approach, where researchers try to explore the process of the occurrence of student interference in solving mathematical problems. The findings of this study are that students experience semantic interference when changing word problem to mathematical models and when interpreting the answers obtained. In addition, students also experience procedural interference when choosing strategies to solve problems, resulting in incorrect answers. Keywords: interference, semantic, procedure 1 Introduction The topic of the proportion taught at junior high school starts from the proportion of two quantities, direct proportion, and inverse proportion. Each of these topics has keywords and characteristics that distinguish each type of proportion. In general, proportion is written in form of a c b d  where a and c are the values of a variable, b and d are the values of other variables [1]–[3]. In addition to this form, a proportion can also be presented in the form of ., 0 y mxm   [4]. Because the presentation is diverse, many students have difficulty understanding the concept of proportion [5]. In addition, the difficulty of students in understanding the concept of proportion is that they relate the concepts of propositions in their daily lives such as: speed, measurement, congruence, geometry, trigonometry [6]–[8]. Many math teachers teach proportions only emphasize the procedure according to what is written in the book. Moreover, math books used in schools contain formal rules for solving problems. Correspondingly, in general teachers only teach proportions by applying cross-multiplication rules to solve proportion problems [6]. When students apply the procedures taught by the teacher, and find the answers, they don't necessarily understand what is written. The use of ICSTI 2018, October 19-20, Yogyakarta, Indonesia Copyright © 2019 EAI DOI 10.4108/eai.19-10-2018.2281319