Three experiments were conducted to investigate the processes of the mathematical problem-solving using juice-mixing problem. The results indicate that the error patterns are not consistent among the problems that have the same mathematical structure, the conceptual bugs in the mixing schemata determine the availability of the values of variables in the problem and the informal knowledge of "thickness" obtained through everyday experiences interacts the subjects' strategy choice. Detailed protocol analysis revealed that the informal knowledge of "thickness" has critical roles to monitor the results and to generate the elaborated internal representations during the course of solving the problem.