Update price + discount

[TheML]

Hello there, I'm working on a web site, where customer select one size of pizza from radio buttons. The size has different prices depending of what radio button customer select. at the bottom, customer will see a text box where price will be display. then if customer is student will receive a 10% off of discount, checking a box and price will be decrease.| My problem is the follow: I don't know how to calculate % off, updating the price.
Javascript:

function updateTotal() 
{
  var sizeAll = document.getElementsByName('product'); 
  var i = sizeAll.length;
  var size = 0;
  var topAll = document.getElementsByName('discount'); 
  var c = topAll.length;
  var top = 0;
  var total = document.getElementById('total');
  
  while(i--) 
  {
   if(sizeAll[i].checked)
   {
   size = parseFloat(sizeAll[i].value);
   }
  }

  if(size > 0)
  {
  
   while(c--)
   {
    if(topAll[c].checked)
	{
	 top += parseFloat(topAll[c].value);
	}
   }
  
  total.value = ((size / 100) * top).toFixed(2);
  }
  else
  {
   total.value = size;
   
  }
 return true;
 }

Html Form:

Pizza Size: (Small 5 Euro- Medium 8 Euro- Large 12 Euro)
	<td id="radios" >
	<input type="radio" name="product"  value="5.00" onclick="updateTotal()">Small(10")
    <input type="radio" name="product" value="8.00" onclick="updateTotal()">Medium(12")
	<input type="radio" name="product" value="12.00" onclick="updateTotal()">Large(14")

Student receive a 10% discount.
	<td id="checkboxes" >
	<input type="checkbox" name="discount" onclick="updateTotal()" value="0.10">

Total Cost:
<input type="text" name="total" id="total" size="10" readonly="readonly">