How to add numbers between a field and a new created field.

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Javascript

please i have a problem here in my work i have and input field and have a button that i use to create new input field with onclick event but my problem is how to add numbers in both input field.

What I have tried:

var count = 1;
function add(){
count++;
var form = document.getElementById('3030')
var divtest = document.createElement("div");

divtest.innerHTML = '<form id = "code_3030"><div id = "credit_unit" name = "myfile"><input type = "text" id = "unit4" placeholder = "Credit" onkeyup = "mul()"></div></form>';

form.appendChild(divtest)
}



function mul(){

var unit = document.getElementById('unit4').value,
output = document.getElementById('code4');

if(form.appendChild(divest) == true){
var ans = unit * divtest;
}else{
alert("work it out");
}

output.value = ans;
}