console
function sumbit(){
var p=parseInt(document.getElementById("one").value)
var u=parseInt(document.getElementById("two").value)
var m=parseInt(document.getElementById("three").value)
var resule=Math.floor(p*18+u*8+m*3)
document.getElementById("four").value=resule
}
function clear_value(){
var input=document.getElementsByTagName("input")
var t;
for (i=0;i<input.length;i++){
input[i].value=" "
}
}
<html>
<head>
<title>麦当劳点餐系统</title>
</head>
<body>
<p>汉堡包：￥18</p>
<input type="number" id="one"><br/>
<p>可口可乐：￥3</p>
<input type="number" id="two"><br/>
<p>薯条：￥8</p>
<input type="number" id="three"><br/>
<button onclick="sumbit()">提交</button>
<button onclick="clear_value()">重置</button><br/>
<p>合计：</p>
<input type="number" id="four">
</body>
</html>