Can't figure out how to solve the decimal point bug in this calculator code

Hi guys, I'm new in flash and is currently learning how to build a simple calculator with multipliers (plus,minus,multiple,divide,decimal point and change sign) but I'm stuck on the decimal point and change sign.

var multiplier_old:Number = 10;
var multiplier_new:Number = 1;
// .: Sets the multipliers so that new input numbers become decimals of a lower unit column
action_point.addEventListener(MouseEvent.MOUSE_DOWN, function():void {
  
          multiplier_old = 1;
          multiplier_new = 0.1;
          point = true;
  
});
// Takes intput from the input_ buttons and adds it to the input after applying the multipliers.
// If `point` is true then the multiplier_new is divided by 10, also as described.
function inputNumber(n:Number):void {
  
          input = input * multiplier_old + n * multiplier_new;
  
                    if (point) {
                                    multiplier_new *= 0.1;
                   trace(multiplier_new);
                    }
  
          output_txt.text = input.toString();
}

Decimal point

The problem is that when I input 2.7 in the calculator, it will display the values in output_txt correctly. But then when I input 2.78, it will display 2.780000000000000000000000002. This will also happen to other numbers if the input is too big.

What I want is just 2.78. How do I change the codings to make 2.780000000000000000000000002 become 2.78?

Change sign

Any tips on how to start on this one?

Thanks for your time,

Zainu