The focus of this study is on a tutorial system in the domain of percentages. We present PerTutor. a system developed under the framework of Intelligent Tutoring Systems. The purpose of the system is to provide reasonable explanations and guidance and to adapt the level of difficulty of problems to each individual's knowledge about percentages. PerTutor is comprised of four main modules: the ex.pen, the student model, the tutor, and the user interface. It relies on previous tutorial interactions to determine both the explanations and the level of difficulty of the problems that will be presented next. An initial implementation of the system demonstrates that PerTutor exhibits some degree of flexibility and intelligence. It generates problems of different levels of difficulty, identifies different ways in which students can approach a problem, and responds according to each situation. Improvements for a future version of the system are also discussed.