What is JotForm?
JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
At JotForm, we want to make sure that you’re getting the online form builder help that you need. Our friendly customer support team is available 24/7.
We believe that if one user has a question, there could be more users who may have the same question. This is why many of our support forum threads are public and available to be searched and viewed. If you’d like help immediately, feel free to search for a similar question, or submit your question or concern.
I am sorry, but there is no way to achieve this on the form builder alone.
OR you can also integrate the form to Google Docs spreadsheet - in the spreadsheet, you can create your formula to do computations on the submission result.
If you can describe more what you wanted to achieve, we can offer more suggestion.