I've just tried a few ways of doing this and found one that works without the NaN error.. I set up a couple of drop down lists all with n/a, 0, 1 & 2 - i put the following rule on the last drop down list that the user will select from:
condition: Rule runs when field changes
Rule type: action
set a fields value: (the text box that you want to have the answer in) to dropdownlist1 + dropdownlist2 + dropdownlist3
hope this helps