How to calculate a user input number by a fixed amount?

How do you calculate a user input number by a fixed amount?

Example:

user selects qty X amount on form

This would be the best guide on how to do it. https://www.jotform.com/help/259-How-to-Perform-Form-Calculation-Using-a-Widget

You can also use Conditional Logic when using calculations https://www.jotform.com/help/268-Conditional-Logic-Inserts-Text-Calculation-Into-A-Field