JSFiddle - React, Tailwind, and code Playground

by Qwerty_Wasd

HTML

<form id="counter"> 
  <input type="radio" name="x" id="2x" value="2" checked>2x
  <br><br>
  <input type="radio" name="x" id="3x" value="3">3x
  <br><br>
  <input name="_1" type="number" placeholder="Операнд 1: ">&nbsp;&nbsp;&nbsp;<input name="_2" type="number" placeholder="Операнд 2: "><br><br>
  <input type="checkbox" id="tov1" value="10">Прибавить 10
  <br><br>
  <input name="res" placeholder="Итого: " readonly>
</form>

JavaScript

document.getElementById(`counter`).addEventListener(`input`, e => {
  e.currentTarget.lastElementChild.value =
    (+e.currentTarget.children[e.currentTarget.children[0].checked ? 0 : 3].value *
     (+e.currentTarget.children[6].value | 0)) +
    (+e.currentTarget.children[7].value | 0) +
    (e.currentTarget.children[10].checked ? +e.currentTarget.children[10].value : 0);
});