Comment implémenter Add (3) .mul (5) .Divide (3) javascript

<!DOCTYPE html>
<html>
<body>

<p>Result is.</p>

<p id="demo">0</p>

<script>
document.getElementById("demo").innerHTML = add(3).mul(5).divide(5);
function add(num) {
	return {
    mul: function(mulNum) {
    		let mulResult = num * mulNum
            return {
            	divide: function(divNum) {
                	let divResult = mulResult/divNum;
                    return divResult
                }
            }
    	}
    }
}
</script>

</body>
</html>
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