Adding up text box numbers with AS3 - actionscript-3

Question

I've been have a heck of a time doing all this. I hope everyone can help.So... what this is doing is taking 9 text box number and adding them up in a dynamic text box. So here are my problems.
How can I replace an empty text box with a 0, if the user gets rid of the 0 that is already in there its will come out NaN. The if statements below were supposed to fix it, maybe someone can improve it.
stage.addEventListener(Event.CHANGE, checkTotal);
nextQuestion_btn.addEventListener(MouseEvent.MOUSE_DOWN, nextQuestion);
function checkTotal(e:Event){
var work:Number = parseInt(work_txt.text);
var rnr:Number = parseInt(rnr_txt.text);
var exerciseB:Number = parseInt(exerciseB_txt.text);
var exerciseM:Number = parseInt(exerciseM_txt.text);
var chores:Number = parseInt(chores_txt.text);
var social:Number = parseInt(social_txt.text);
var food:Number = parseInt(food_txt.text);
var twt:Number = parseInt(twt_txt.text);
var partying:Number = parseInt(partying_txt.text);
var other:Number = parseInt(other_txt.text);
if(work_txt.text==""){
work=0;
}
if(rnr_txt.text==""){
rnr=0;
}
if(exerciseB_txt.text==""){
exerciseB=0;
}
if(exerciseM_txt.text==""){
exerciseM=0;
}
if(chores_txt.text==""){
chores=0;
}
if(social_txt.text==""){
social=0;
}
if(food_txt.text==""){
food=0;
}
if(twt_txt.text==""){
twt=0;
}
if(partying_txt.text==""){
partying=0;
}
if(other_txt.text==""){
other=0;
}
var total400:Number = work + rnr + exerciseB + exerciseM +
chores + social + food + twt + partying + other;
I can't let my text boxes add up over 400 so as the user types in 399 into one box, if the user types 2 into the next that current text box will revert to 0 because it would be over 400.
I was told using e.currentTarget could solve that problem but I'm not sure how to use it.
All my code...This is my first time on this site so please forgive me for my noobness.
work_txt.maxChars = 3;
rnr_txt.maxChars = 3;
exerciseB_txt.maxChars = 3;
exerciseM_txt.maxChars = 3;
chores_txt.maxChars = 3;
social_txt.maxChars = 3;
food_txt.maxChars = 3;
twt_txt.maxChars = 3;
partying_txt.maxChars = 3;
other_txt.maxChars = 3;
work_txt.restrict = "0-9"
rnr_txt.restrict = "0-9"
exerciseB_txt.restrict = "0-9"
exerciseM_txt.restrict = "0-9"
chores_txt.restrict = "0-9"
social_txt.restrict = "0-9"
food_txt.restrict = "0-9"
twt_txt.restrict = "0-9"
partying_txt.restrict = "0-9"
other_txt.restrict = "0-9";
/*work_txt.text = "0";
rnr_txt.text = "0";
exerciseB_txt.text = "0";
exerciseM_txt.text = "0";
chores_txt.text = "0";
social_txt.text = "0";
food_txt.text = "0";
twt_txt.text = "0";
partying_txt.text = "0";
other_txt.text = "0";*/
var survival:Number = 0;
nextQuestion_btn.visible=false;
stage.addEventListener(Event.CHANGE, checkTotal);
nextQuestion_btn.addEventListener(MouseEvent.MOUSE_DOWN, nextQuestion);
function checkTotal(e:Event){
var work:Number = parseInt(work_txt.text);
var rnr:Number = parseInt(rnr_txt.text);
var exerciseB:Number = parseInt(exerciseB_txt.text);
var exerciseM:Number = parseInt(exerciseM_txt.text);
var chores:Number = parseInt(chores_txt.text);
var social:Number = parseInt(social_txt.text);
var food:Number = parseInt(food_txt.text);
var twt:Number = parseInt(twt_txt.text);
var partying:Number = parseInt(partying_txt.text);
var other:Number = parseInt(other_txt.text);
if(work_txt.text==""){
work=0;
}
if(rnr_txt.text==""){
rnr=0;
}
if(exerciseB_txt.text==""){
exerciseB=0;
}
if(exerciseM_txt.text==""){
exerciseM=0;
}
if(chores_txt.text==""){
chores=0;
}
if(social_txt.text==""){
social=0;
}
if(food_txt.text==""){
food=0;
}
if(twt_txt.text==""){
twt=0;
}
if(partying_txt.text==""){
partying=0;
}
if(other_txt.text==""){
other=0;
}
var total400:Number = work + rnr + exerciseB + exerciseM +
chores + social + food + twt + partying + other;
trace(work);
trace(rnr);
trace(exerciseB);
trace(exerciseM);
trace(chores);
trace(social);
trace(food);
trace(twt);
trace(partying);
trace(other);
trace(total400);
total400_txt.text = String(total400);
if(total400 >= 400){
nextQuestion_btn.visible=true;
}else{
nextQuestion_btn.visible=false;
}
}

Answer 1

Q1
If the values only can be int type, try to use int instead of Number
var work:int = parseInt(work_txt.text);//work will be 0 if the text is empty
Q2
If you want revert the text to 0(the text box which input 2)
function checkTotal(e:Event){
var target:TextField = e.target as TextField;
if(total400 >= 400){
if (target) {//you may check if target is one of the text box you have listed.
target.text = "0";
}
nextQuestion_btn.visible=true;
}
}