Calculating total based on user selections

How would I create a condition that enables the information put on my particular form (adults over the age of 18 to calculate the value of the amount of the application fee), ex: adults over 18=2, to make the application fee quantity to equal 2

This is not possible to dynamically select a specific product quantity based on other field selection.

1) The alternative is to calculate a total value.

 How to Perform Calculation in the Form

Example:

2) Then, pass it to the payment field.

How to pass a calculation to a payment field

Example:

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