JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.4.1/jquery.min.js"></script>
<input type="number" name="nb" id="nb" />
<button id="calculer">Calculer</button>
<div id="poids" style="display: none;">
  <span></span>g
</div>

JavaScript

$('#calculer').click(function(){
  var valeur = $("#nb").val();
  if(valeur != 0)
  	$('#poids').show().find('span').text(valeur*5)
  else
  	$('#poids').hide();
});