JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" id="box1" oninput="calculate()" />
<input type="text" id="box2" oninput="calculate()" />
<input type="text" id="box3" oninput="calculate()" />

<input id="result" />

<br /><br /><br />

<input type="checkbox" name="changeValue" id="changeValue" />
<label for="changeValue">Divide with 35</label>

JavaScript

function calculate() {
    var myBox1 = document.getElementById('box1').value;	
    var myBox2 = document.getElementById('box2').value;
    var myBox3 = document.getElementById('box3').value;
    var result = document.getElementById('result');	
    var myResult = myBox1 * myBox2 * myBox3 / 25;
    result.value = myResult;    
}