JSFiddle - React, Tailwind, and code Playground

HTML

<form name="form1" id="form1">
<div>
  <label>Quantidade:</label>
  <input type="text" name="qtd1" id="qtd1" onBlur="chama_onChange()" value="0">
</div>
<div>
  <label>Preco:</label> 
  <input type="text" name="pc1" id="pc1" onBlur="chama_onChange()" value="0">
</div>
<div>
  <label>Total:</label>
  <input type="text" name="total1" id="total1" disabled>
</div>
</form>

CSS

label {
  display:table;
}

JavaScript

function chama_onChange()
{
		var qtd1 = document.getElementById("qtd1");
		var pc1 = document.getElementById("pc1");
		var total1 = document.getElementById("total1");
		
		if (isNumber(qtd1.value) && isNumber(pc1.value))
		{
			total1.value = parseFloat(qtd1.value) *  parseFloat(pc1.value);
		} 
		else
		{
			total1.value = 0;
		}
}

function isNumber(n) {
    return !isNaN(parseFloat(n)) && isFinite(n);
}