You can validate the Multiple Textboxes question type to only allow numerical data, and require all fields to add up to a fixed sum. This field will only accept positive, whole numbers.
To validate a Multiple Textboxes question to require a fixed numerical sum:
TIP! If you require a fixed sum, be sure to mention the required total in the question text itself to ensure that respondents know how to properly answer the question the first time.
A common use of this feature is to ask respondents to provide answers in percentages that add up to 100%. However, the numerical data field will not accept percentage signs. We suggest adding question text instructing respondents to omit the percent sign (%) from their responses.
Respondents will not see a running total update dynamically as they enter their answers, but if they enter a set of numbers that do not add up to the required fixed sum, they will receive an error message when they try to move to the next page of the survey, indicating that the choices must add up to a specific number.