Does solving subtraction problems with negative answers (e.g., 5-14) require different cognitive processes than solving problems with positive answers (e.g., 14-5)? In a dual-task experiment, young adults (N=39) combined subtraction with two working memory tasks, verbal memory and visual-spatial memory. All of the subtraction problems required verbal working memory but only large problems with negative answers (e.g., 8-17) required visual-spatial working memory. Small-operand problems (e.g., 5-3) required less verbal working memory than large-operand problems (e.g., 15-9). Answers to small problems were probably retrieved from memory even when the answer was negative (e.g., 3-5). In contrast, large problems with negative answers may have required participants to modify their solution procedures such that problem difficulty increased. These results indicate that even relatively simple subtraction problems can be cognitively demanding of both verbal and visual-spatial working memory, especially when solutions are not activated automatically.