multiple answers with a fixed sum (probabilties that sum up to 100%)

[Duk Gyoo Kim]

Dear LIONESS team,

I'd like to know what would be the best way to collect multiple answers with a fixed sum.
For example, suppose that I want to ask how a respondent would spend $100 for three different items.
- It would be ideal if the respondent types two numeric inputs, say $20 and $54 for the two items, then $26(=100-20-54) for the last item would be automatically calculated and displayed.
- If I create three elements taking numeric input from 0 to 100, then I cannot make sure that the sum of the answers is 100. Also, I find it redundant to ask them to fill out all three questions.

For now, I consider adding a "validity check" stage after collecting the answers, displaying a message that the respondent can move on to the next stage if the answers are legit, and an error message to ask them to go back to the previous stage otherwise. I don't think this is the best way. Please advise me.

After a quick search, I realize that typical survey platforms provide such a feature:
Qualtrics https://www.qualtrics.com/support/survey-platform/survey-module/editing-questions/question-types-guide/specialty-questions/constant-sum/
SurveyMonkey https://help.surveymonkey.com/en/surveymonkey/create/running-total/

I hope the two links above may help to clarify what I want to do.

Taking advantage of contacting you, I'd like to express how much I appreciated LIONESS. Thank you so much. I look forward to hearing from you.

Sincerely,

Duk