Solving arithmetic word problems requires individuals to create a correct mental representation, and this involves both text processing and number processing. The latter comprises understanding the semantic meaning of numbers (i.e., their magnitudes) and potentially executing the appropriate mathematical operation. However, it is not yet clear whether number processing occurs after text processing or both take place simultaneously. We hypothesize that number processing occurs early and simultaneously with other problem-solving processes such as text processing. To test this hypothesis, we created non-solvable word problems that do not require any number processing and we manipulated the calculation difficulty using carry/borrow vs. non-carry/non-borrow within addition and subtraction problems. According to a strictly sequential model, this manipulation should not matter, because when problems are non-solvable, no calculation is required. In contrast, according to an interactive model, attention to numbers would be higher when word problems require a carry/borrow compared to a non-carry/non-borrow operation. Eye-tracking was used to measure attention to numbers and text in 63 adults, operationalized by static (duration and count of fixations and regressions) and dynamic measures (count of transitions). An interaction between difficulty and operation was found for all static and dynamic eye-tracking variables as well as for response times and error rates. The observed number processing in non-solvable word problems, which indicates that it occurs simultaneously with text processing, is inconsistent with strictly sequential models.