How do I calculate multiple fields?

[needadobehelp56]

I have a form that uses radio buttons to populate a subtotal field. The subtotal field is a text field as I can't get it to work with a numeric field. Below the subtotal is a field for tax and then total. How do calculate the total using the value from subtotal and tax?

[Jono_Moore]

It's going to be more difficult to do if you're not using numeric fields.

Why aren't the numeric fields working?