A computer model of memory (based on input, retrieval, and reaction time) was applied to the rapid processing of simple arithmetic facts (addition, subtraction, and multiplication) by 18 educable retarded adolescents when compared with 18 nonretarded fourth graders (of equal mental age) and 18 normal adolescents. Results for addition indicated that all three groups use a digital (rather than analog) processing model with the faster reaction time of the normal adolescents attributable not to memorization but faster perceptual and motor processes. Results for subtraction supported the digital model for the retarded and young Ss but suggested that the normal adolescents used a memory retrieval system. Results for multiplication also supported the digital model for all three groups indicating that addition and multiplication are based on the same common mental processing mechanism. Results support educational practices of encouraging retarded children to manipulate objects and actively participate as they learn arithmetic facts. (Appended are a list of terms with definitions and statistical tables.) (DB)