JSFiddle - React, Tailwind, and code Playground

HTML

<form action="fichier.php" name="form" >
 	<input type="number" min=0 name="appels" />
 	<input type="number" min=0 name="reponses" />
 	<input type="text" name="resultat" />
 </form>

JavaScript

function result(){

	var numApp = parseInt(app.value),
		numRep = parseInt(rep.value),
		resultat = (numRep / numApp) * 100;
	
	!isNaN(resultat) && (document.form.resultat.value = resultat);
}

var app = document.form.appels,
	rep = document.form.reponses;

app.addEventListener("change", result, false);
rep.addEventListener("change", result, false);