JSFiddle - React, Tailwind, and code Playground

HTML

absolut value:<input type="text" id="absolut" value="0"/>
percentage of 5:<input type="text" id="percentage" value="0"/>

JavaScript

$("#absolut").change(function(){
    $('#percentage').val(parseFloat($(this).val())*100/5);
});

$("#percentage").change(function(){
    $('#absolut').val(parseFloat($(this).val())*5/100);
});