JSFiddle - React, Tailwind, and code Playground

HTML

<form name="form">
 
    <select name="horas"  onchange="calcula_porcentaje(this.value)">
    <option value="" disabled selected>Cantidad Horas:</option>
    <option value="5">5 Horas</option>
        <option value="3">3 Horas</option>
    </select><br>
     
<input id="icon_prefix" class="black-text" type="text" name="valor_horas" value="3" readonly/><label for="cuentas" class="black-text ">Valor Horas:</label>
 
 <br>
<input id="icon_prefix" class="black-text" type="text" name="total_pago"/>
<label for="cuentas" class="black-text ">Total:</label>

    </form>

JavaScript

function calcula_porcentaje(horas) {
  //alert(horas);
  var valor_horas = document.form.valor_horas.value;
  var total = horas * valor_horas;
  document.form.total_pago.value = total;
}