How to use Form Calculation Widget?

No, I was talking about something else that I used once in my form.

It is a calculator that allows to summarize the chosen options.

For example if one class has been chosen, it is one price, if two classes have been marked, the double price appears automatically.

Thank you!

I assume you are referring to this guide: How-to-Perform-Form-Calculation-Using-a-Widget

You can achieve your mentioned calculation with Form Calculation Widget:

As you could see in the above figure, the total amount is calculated based on the choices. 

If you need any further assistance, please let us know.