JSFiddle - React, Tailwind, and code Playground

HTML

<div id="container">
<form action="something.html" id="subit">
<label for="first">Post nr </label><input type="text" id="postnr"/>
<label for="first">Kvm </label><input type="text" id="first"/>

</form>
    <p>Pris</p><div id="result">1500</div>
</div>

JavaScript

$(document).ready(function() {
	var post = $("#postnr");
  var postval = $("#postnr").val();
  
  if (postval == 100){

    var textboxes = $("#first");
    textboxes.on("keyup", function() {
        var amt = 1500;
        textboxes.each(function() {
            amt += +$(this).val();
        });
        $("#result").html(amt);
    });
};
	
  else {
  	$("#result").html("fan");
  };
});