A-B=C C must be positive number

Hi,

That's possible. Here, I have created you an example form.

http://form.jotform.co/form/40028949160858?

Feel free to clone this form to copy it in your account.

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To view the calculation formula on the field "C", just click that field and click Calculate.

To view the existing conditions, check this visual guide:

Let us know if you have further questions or if you need further assistance about this.

Regards!