รบกวนถามเรื่องระบบ คำนวณหน่อยครับ เช่นมีการตัดเงินมัดจำ บวกภาษี หักส่วนลด

<script>
$(document).ready(function() 
{
	var mainprice = $('#main_price').val();
	
	$('#tax_percent').change(function(){
		percent = parseFloat(mainprice)*parseFloat($(this).val())/100;
		total = parseFloat(mainprice)+percent;
		
		$('#tax').val(percent);
		$('#tax_result').val(total);
		check();

	});
	
	$('#discount_percent').change(function(){
		price = $('#tax_result').val();
		percent = parseFloat(price)*parseFloat($(this).val())/100;
		total = parseFloat(price)-percent;
		
		$('#discount').val(percent);
		$('#discount_result').val(total);
		check();

	});
	
	$('#pledge_percent').change(function(){
		price = $('#discount_result').val();
		percent = parseFloat(price)*parseFloat($(this).val())/100;
		total = parseFloat(price)-percent;
		
		$('#pledge').val(percent);
		$('#pledge_result').val(total);
		check();

	});
	
	function check()
	{
			
			var tax = $('#tax').val();
			var discount = $('#discount').val();
			var pledge = $('#pledge').val();
			
			price_total = (parseFloat(mainprice)+parseFloat(tax))-parseFloat(discount)-parseFloat(pledge);
			$('.test').html('mainprice: '+mainprice+'<br>tax: '+tax+'<br>discount: '+discount+'<br>pledge: '+pledge+'<br>total: '+price_total+'<br>');
			 $('#total').val(price_total);
	}
	

	
});
</script>


<input type="text" class="form-control" name="main_price" id="main_price"  value="5900">

<div class="form-group">
  <label for="tax_percent" class="col-sm-2 control-label">ภาษี</label>
  <div class="col-sm-2">
    <select name="tax_percent" id="tax_percent" class="form-control"> 
    <? 
        for($i=0;$i<=7;$i++) {	
    ?>
      <option value="<?=$i?>"><?=$i?>%</option>
    <? 
    } ?>  
    </select>
  </div>
  <div class="col-sm-2">
    <input type="text" class="form-control" name="tax" id="tax" placeholder="0.00">
    <input type="text" name="tax_result" id="tax_result" value="" />
  </div>  
  <div class="col-sm-1">
    บาท
  </div>                                            
</div> 

<div class="form-group">
  <label for="discount_percent" class="col-sm-2 control-label">ส่วนลด</label>
  <div class="col-sm-2">
    <select name="discount_percent" id="discount_percent" class="form-control">
    <? 
        for($i=0;$i<=100;$i=$i+5) {	
    ?>
      <option value="<?=$i?>"><?=$i?>%</option>
    <? 
    } ?>  
    </select>
  </div>
  <div class="col-sm-2">
    <input type="text" class="form-control" name="discount" id="discount" placeholder="0.00">
    <input type="text" name="discount_result" id="discount_result" value="" />
  </div>  
  <div class="col-sm-1">
    บาท
  </div>                                            
</div> 

<div class="form-group">
  <label for="pledge_percent" class="col-sm-2 control-label">มัดจำ</label>
  <div class="col-sm-2">
    <select name="pledge_percent" id="pledge_percent" class="form-control" >
    <? 
        for($i=0;$i<=100;$i=$i+10) {	
    ?>
      <option value="<?=$i?>"><?=$i?>%</option>
    <? 
    } ?>  
    </select>
  </div>
  <div class="col-sm-2">
    <input type="text" class="form-control" name="pledge" id="pledge" placeholder="0.00" >
    <input type="text" name="pledge_result" id="pledge_result" value="" />
  </div>  
  <div class="col-sm-1">
    บาท
  </div>                                            
</div> 

<div class="form-group">
  <label for="total" class="col-sm-2 control-label">ยอดสุทธิ</label>
  <div class="col-sm-2">
    <input type="text" class="form-control" name="total" id="total" placeholder="0.00">
    <input type="text" name="total_result" id="total_result" value="" />
  </div>    
  <div class="col-sm-1">
    บาท
  </div> 
  <span class="test"></span>   
</div>