This study investigated the effectiveness of using the Calcumate, a copyrighted commercially available product, in teaching subtraction to average, disturbed, and retarded children in third, fourth, and fifth grades. The mechanical device simulates borrowing in subtraction problems involving two-digit numbers. Forty students were given these devices for introductory work with subtraction; they were told to use the aids only as long as they needed them. The instructional phase lasted 30 days, with students performing subtraction problems each day. Third-grade students were given 20 problems to perform in 30 minutes; other subjects were given 10 problems and 15 minutes. In all groups, substantial gains in ability to subtract with borrowing were recorded. Eighty-five days after the special instruction ended, students were retested on subtraction without using their Calcumates, and achievement continued to be high. (SD)