This study was designed to investigate the effectiveness of a heuristic approach to problem-solving instruction. The 330 subjects were randomly assigned to either a Problem Solving, Calculator Problem Solving, or Control group. The two experimental groups studied problem-solving heuristics applied to nonroutine problems for 18 weeks, after which 102 subjects of 3 ability groups were randomly selected for post testing. In an interview setting they responded to five-selected problems. Seven subjects were identified as case studies. Results showed that the experimental groups had significantly higher problem-solving success and used more strategies. The Calculator Problem Solving subjects were found to make fewer computational errors and required significantly less time in problem solving. (Author/MP)