Understanding Conditional Radio Buttons

Hope you can help. Trying to write a form using Adobe Designer.
 
I have a question in the form requiring a 'yes'/'no' answer. Depending upon which 'radio button' the user selects, a different calculation will extend into a 'subtotal column'.
 
For example:
 
Q: Do you have a low-water flow showerhead?
A: Yes (radio button) No (radio button)
 
What happens as a result of selecting 'yes' is that answer derived from a previous question (number of people in household) is multiplied by another question (average number of minutes in shower per week by all family members) to give a result multiplied by 7.5 (number of litres per minute a water-efficient shower head uses).
 
If they answer 'no', then the result will show a multiplication by 12 instead of 7.5.
 
Ok, now my dilemma is understanding if/else so that I can have a FormCalc script to change the resulting totals dependent upon whether a user selects 'yes' or 'no' using radio buttons to various questions like these.
 
I hope all that makes sense to you.
 
The bottom line is: I'd love some help in simple terms as to how to write the necessary if/else script.
 
Can you help me please?
 
Thanks.