The objective of this practicum was to apply mastery learning principles in the development of a computer-based instruction lesson on "Divisibility Rules," which was designed for students preparing for the General Education Development (GED) examination. This practicum sought to demonstrate that computer-based instruction which follows mastery learning principles facilitates student learning and fosters positive student attitudes toward learning. The lesson on "Divisibility Rules" follows a systematic approach to instruction that offers the student a rationale, objectives, pretest, alternative learning activities, and posttest with provision for revision. PLATO (Programmed Logic for Automatic Teaching Operations) is a computer-oriented instructional system that allows instructors to design individualized lessons for their students. The achievement results of the target group failed to measure up to the goal that 80 percent would achieve mastery. Technical difficulties hampered the results. The attitudinal results, however, were unanimously positive; this demonstrates that mastery learning strategies can provide students with enjoyable learning experiences. (MJK)