<input type="text" id="txt1" value="123" /> 
<input type="text" id="txt2" value="123" /> 
var num1= $("#txt1").val(); 
var num2 = $("#txt2").val() 
var total = num1+num2;

<center>
    <div class="table-row-group">  
		<ul class="table-row">
		    <li style="border: 0px;font-size: 35px;">剩余</li>
		    <strong>
			<li id="subtraction1" style="border: 0px;font-size: 35px;">0</li>
			</strong>
		    <li style="border: 0px;font-size: 35px;">收入</li>
			<strong>
			<li  id="subtraction2" style="border: 0px;font-size: 35px;">5478.28</li>
			</strong>
		    <li style="border: 0px;font-size: 35px;">支出</li>
			<strong>
			<li  id="subtraction3" style="border: 0px;font-size: 35px;">3304.70</li>
			</strong>
		</ul>
		
    </div>
</center>
<script language="javascript">
	function subtraction(first,second){ //兩個數(shù)相減，相除，相乘 - / *,加法要這樣 first*1 + second*1
	var results = first - second;
	results = results.toFixed(2);
	return results;
}
	$("#subtraction1").text(subtraction($("#subtraction2").text(),$("#subtraction3").text()));
</script>