<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width">
</head>
<body>
<div id="result"></div>
<button onClick="setNumber(this)" value="20">20</button>
<button onClick="setNumber(this)" value="30">30</button>
<button onClick="setNumber(this)" value="40">40</button>
<script>
var result = document.getElementById('result')
var num = 0;
function setNumber(e){
result.innerHTML = num += parseInt(e.value);
}
</script>
<style>
#result{
border:1px solid green;
width:100px;
height:50px;
margin-bottom:10px
}
</style>
</body>
</html>