JSFiddle - React, Tailwind, and code Playground

HTML

Pixels <input type="text" name="mytext[]" id="Pixels" value="Text 1"></input>
    <br />
    Percentage <input type="text" name="mytext[]" id="Percentage" value="Text 1"></input>

JavaScript

var X = 80;
$('#Pixels').keyup(function(){
    
    if(!isNaN(this.value)){
       $('#Percentage').val(parseInt(this.value)/X *100)
    }
});

$('#Percentage').keyup(function(){
    $('#Pixels').val(parseInt(this.value) * X/100);
    
});