Multiple Textboxes question can be validated to only allow numerical data and require all fields to a fixed sum. Those fields will only accept positive whole numbers.

Validating Multiple Textboxes question to only allow numerical data and require a fixed sum:
1. Go to the Design Survey tab.
2. Select a Multiple Textboxes question to edit it. Or add a new Multiple Textboxes question type from the BUILDER section of the left sidebar.
3. Below the Textboxes fields, tick the next to Only Allow Numerical Data.
4. Then tick the next to Require a Fixed Sum.
5. In the Sum of all Answers = field, enter the number that all answers must add up to.
6. Modify the error message to be clear about the requirements of the question.
7. Click Save.

Note: If you require a fixed sum, make sure to mention the required total in the question text itself to ensure that respondents know how to answer the question correctly.