Something more like this but this is buggy with your buttons sometimes, not entirely sure, not really looked much at the file.

Actionscript Code:
stop();

stotal=0;
a1a=false;
a1b=false;

q1a.onRelease = function(){
    if (a1a==true){
        a1a=false;
        stotal -=75;
    }else{
        a1a=true;
        stotal += 75;
    }
}

q1b.onRelease = function(){
    if (a1b==true){
        a1b=false;
        stotal -=50;
    }else{
        a1b=true;
        stotal += 50;
    }
}

also I changed your txtfield var to stotal.