JSFiddle - React, Tailwind, and code Playground

HTML

<span>Set new quantity: </span>
<span>10 + </span>
<input id='diff' type='text' value='0' /><span> = </span>
<input id='result' type='text' value='10' />

JavaScript

const initialQty = 10

document.querySelector('#diff').addEventListener('change', (e) => {
  document.querySelector('#result').value = initialQty + Number(e.target.value)
})

document.querySelector('#result').addEventListener('change', (e) => {
  document.querySelector('#diff').value = Number(e.target.value) - initialQty
})