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HTML Document File : percentage.html 

<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <title>percentage</title>
</head>
<body>

<p>
inputs:
<input class="input" type="number" id="input_number" placeholder="input number">

<span style="white-space: nowrap;"><input class="input" type="number" id="percentage" placeholder="percentage">%</span>
</p>

<p>
applied percentage:
<input type="number" id="applied" placeholder="applied percentage">
</p>

<script>
for(var input of document.querySelectorAll(".input")) input.oninput = apply
function apply() {
    applied.value = input_number.value * percentage.value / 100
}
</script>

</body>
</html>