JSFiddle - React, Tailwind, and code Playground

HTML

<select id="quantity">
    <option selected="selected" value="1">1</option>
    <option value="2">2</option>
    <option value="3">3</option>
    <option value="4">4</option>
  </select>

  <div id="here"></div>

JavaScript

$("#quantity").change(function () {
     var str = $(this).val();
     $("#here").text(parseInt(str, 10) * 10);
 }).change();