JSFiddle - React, Tailwind, and code Playground

HTML

<input id="elem1" type="text" name="quantity" placeholder="Количество">
<div id="str"></div>

JavaScript

elem1.oninput = function() {
		var val = document.getElementById('elem1').value;
		document.getElementById('str').innerHTML="Вы ввели: "+val*0.3;
	};