The mastery of basic fundamental knowledge and skills is emphasized in this new course of study. When teaching systematically learned courses like arithmetic, it is important to verify the pupil's thorough mastery of the basic knowledge and skills, or else the pupil will stumble in understanding higher math and find it difficult to catch up. This study offers a curriculum plan for teaching single-digit addition based upon a gradual mastery of simple to more difficult problem-solving based on the relative difficulty of the problem. First, the researchers re-examined the results of previously conducted studies concerning the relative difficulty of the problem solving for pupils. Then, a new plan of curriculum based on the approach of relative difficulty of problem solving was designed in which pupils systematically learn arithmetic by a gradual mastery of simple to more difficult problem-solving. Finally, based on those new criteria, the problems contained in two textbooks were compared to see which problems would be most appropriate.