Numerous controlled studies prove the effectiveness of Intelligent Tutoring Systems (ITSs). But what happens when ITSs are available to students for voluntary practice? EER-Tutor is a mature ITS which was previously found effective in controlled experiments. Students can use EER-Tutor for tutored problem solving, and there is also a special mode allowing students to develop solutions for the course assignment without receiving feedback. In this paper, we report the observations from two classes of university students using EER-Tutor. In 2018, the system was available for completely voluntary practice. We hypothesized that the students’ pre-existing knowledge and the time spent in EER-Tutor, mediated by the number of attempted EER-Tutor problems, contribute to the students’ scores on the assignment. All but one student used EER-Tutor to draw their assignment solutions, and 77% also used it for tutored problem solving. All our hypotheses were confirmed. Given the found benefits of tutored problem solving, we modified the assignment for the 2019 class so that the first part required students to solve three problems in EER-Tutor (without feedback), while the second part was similar to the 2018 asignement. Our hypothesized model fits the data well and shows the positive relationship between the three set problems on the overall system use, and the assignment scores. In 2019, 98% of the class engaged in tutored problem solving. The 2019 class also spent significantly more time in the ITS, solved significantly more problems and achieved higher scores on the assignment.