Limited systematic instruction was provided for two third-grade classes on a calculator-assisted approach to solving selected types of simple open addition and subtraction sentences involving three-place whole-number addends, moving from a Guess-and-Test procedure to more direct and efficient sentential transformations. Pupils exhibited a relatively high level of computational accuracy in their use of minicalculators or hand-held electronic calculators (94%) but substantially lower levels of proficiency in providing mathematically correct solutions for "taught" and related "untaught" open-sentence types (ranging from 66% to 23%). Serious questions are raised regarding the appropriateness for young children of certain instructional approaches to the solution of simple open addition and subtraction sentences. (Author/SD)