That depends on where you are desiring an equation. Do you wish to have some calculation in your form? We do not currently support calculations. If you want to add some equations to the View Responses table to calcululate data from your responses you can edit the table adding additional columns and add equations there. See the "Use Formulas in a column" section here:
I have a field in my form for the number of guest and it's 20$ per guest. Can it multiple ?
or in my response table can I multiple one colonne with a other ?
For Paypal payments you can use a "Quantity" field - see this FAQ on Paypal and review the "Quantity Field" section: http://forums.adobe.com/docs/DOC-1632
Hopefully this gives you enough guidance.