JSFiddle - React, Tailwind, and code Playground

HTML

<form action="#">
  <span id="main">100</span>
  <fieldset>
    <label for="summandOne">Слагаемое 1</label>
    <select name="summandOne" id="summandOne">
      <option value="200">200</option>
      <option value="300">300</option>
      <option value="400">400</option>
    </select>
  </fieldset>
   <fieldset>
    <label for="summandTwo">Слагаемое 2</label>
    <select name="summandTwo" id="summandTwo">
      <option value="500">500</option>
      <option value="600">600</option>
      <option value="700">700</option>
    </select>
  </fieldset>
</form>

CSS

#main{
  display: block;
  padding: 2em;
  font-size: 1.5em;
  font-weight: 700;
  color: red;
}

JavaScript

summandOne.onchange = summandTwo.onchange = function(){
   main.innerHTML = +summandOne.value + (+summandTwo.value)
}