Solving arithmetic word problems requires individuals to create a correct mental model, and this process involves both text processing and number processing. The latter involves selecting and executing the appropriate mathematical operation. However, it is not yet clear whether number processing is part of building the mental model, and whether it occurs after text processing or simultaneously with other problem-solving processes. We hypothesize that number processing is an early process that occurs simultaneously with other problem-solving processes. To test this hypothesis, we created non-solvable word problems that do not require any number processing and manipulated the calculation difficulty using carry/borrow and non-carry/non-borrow addition/subtraction operations. In a sequential model, this manipulation should not matter, because the mental problem model suggests that the problem is not solvable, and no calculation is required. In contrast, according to an interactive model, attention to numbers would be higher when word problems required a carry/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. That number processing is observed in non-solvable word problems suggests early number processing occurs simultaneously with text processing and other problem-solving processes, which is inconsistent with strictly sequential models.