This paper is a study on solving combinatorial explosion problems that occur when students combine multiple steps into one in an intelligent tutoring system that supports step-by-step problem solving. In order to overcome the limitations of the existing method of analyzing the connection between continuous inputs as a single rule, we propose an error diagnosis approach using the final answer. We designed a service that diagnoses error rules based on the final answer, and evaluated its performance by applying it to the quadratic equation solving process (n=1939) dataset. As a result, 29.4% of the steps could be diagnosed through the final answer evaluation, and the agreement rate with the teacher diagnosis (n=115) was 97%.