JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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The updates in the Calculation wiget allows you to build your formulas with no need to type any digit on it, all you need to do is to click on the needed buttons. Please see this gif screenshot:
I hope this helps. Please inform us if you need further assistance with this inquiry.
Here is also a guide that will explain you with more details how to use it: How to perform calculation in the form