Adobe Designer 7.0 Formcalc

I am using Adobe Designer 7.0, which I know is outdated, but it meets my needs.  I have a PDF form with a two-radio button field - 4.0 and 5.0.  A scholarship applicant enters his/her GPA, and clicks the appropriate scale.  If the GPA is on the 4.0 scale, the CALCGPA field displays the GPA the student entered.  If, however, the student clicks the 5.0 radio button, a calculation multiplying the GPA by 4/5 to set the 5.0 scale GPA just entered equal to the 4.0 scale.  I need help with how to do this in Formcalc.