function startCalc(element_num){
 var tmp = element_num;
  interval = setInterval( "calc('"+ tmp +"')", 1);
}
function roundVal(val){
	var dec = 2;
	var result = Math.round(val*Math.pow(10,dec))/Math.pow(10,dec);
	return result;
}

function calc(element_num){
  price = document.getElementById("price" + element_num).value;
  broi = document.getElementById("br" + element_num).value; 
  discount = document.getElementById("discount" + element_num).value; 
  tmp_sum = parseFloat(price) * parseFloat(broi);
  if (discount==0){
  	total_price = tmp_sum;
  }else{
	  tmp_discount =  (tmp_sum * parseFloat(discount)) / 100;
	  total_price = tmp_sum - tmp_discount;
 }
  if(isNaN(total_price)){
  	total_price = 0;
  }
  
  document.getElementById("sum" + element_num).value = roundVal(total_price);
}
function stopCalc(){
  clearInterval(interval);
}