Prepopulating Fields with Numeric Values




I have a form which has a First Rental Box which our customers type in a numeric value. Next to this box is a VAT box which needs to be populate automatically. VAT is calculated by First Rental Value * 17.5 / 100. How I automatically populate the vat box with the correct value? Also, once thats been done, I have a total box which needs to be populate with First Rental + VAT?

Hope you guys can help.