Adding variables

I have two comboboxes on my stage,
I used this code for one:
on (change) {
var items = (value)
var fountain:Number = (items * 50);
_parent.fsum.text = fountain;

}
This for the other:

on (change) {
var items = (value)
var water:Number = (items * 50);
_parent.wsum.text = water;

}

How do I code it to get the total for both comboboxes? And, do I put the code on an action layer?