JSFiddle - React, Tailwind, and code Playground

HTML

Price : <input type="text" class="price" value="10000"><br/>

Tax (%) : <input type="text" class="dp_per" value="5"><br/>

Tax Amount : <input type="text" class="dp_per" value=""><br/> <br/>

JavaScript

var price = document.getElementsByClassName('price')[0];
var tax = document.getElementsByClassName('dp_per')[0];
var taxAmount = document.getElementsByClassName('dp_per')[1];

tax.onblur = function() {
    taxAmount.value = parseFloat(price.value) * parseFloat(tax.value) / 100;
}