Drop down menus have a display value and an export value. If the export value is not entered, it becomes the display value by default. The export value is the 'value' used in calculations. Therefore, enter 0 (zero) as the export value of N/A, and N/A will count as zero when using this field in a numeric calculation.
Second question:
First count the fields that are not N/A (zero), then divide by the sum of all the dropdowns that are not zero. Assuming you a consistent naming convention of dropdown1, dropdown2,.....dropdown5:
var num=0;
var ddvalue=0;
for (var i=1;i<6;i++)
{
if (this.getField("dropdown"+i).value!=0)
{
num=num+1;
ddvalue=ddvalue + this.getField("dropdown"+i).value;
}
}
event.value=ddvalue/num;