This study aims to describe students' thinking processes in solving mathematical problems based on Krulik and Rudnick's rules. This research is qualitative research with a case study method. Sampling was done by a combination of purposive sampling and snowball sampling. The research subjects used in this study were nine subjects, namely: three high-ability students, three medium-ability students, and three low-ability students. Data were validated by using the time triangulation test. The results showed: (1) In the reading and thinking step, students immediately identified all the facts in writing. After identifying the facts, students identify questions from the given problem. (2) In the exploration and planning stages, students organize information about the material. Students consider sufficient information about the material because there is a relationship between what is known and what is asked. Students immediately choose strategies to answer questions; (3) In the strategy selection step, students use simulation or experimental strategies, students try to experiment with certain theorems; (4) In the step of finding the answer, students use estimation in finding the answer. On the other hand, there is one student who uses algebraic skills to find the answer. (5) In the reflection and expansion step, students can review the calculation results by returning them to the answer sheet. Then students find that the answer is correct.