calcul du total

bonjour à tous,
je souhaite afficher le total dans une zone texte en fonction du choix dans mes listes. ex: je suis un menu (na) si je choisis b2€ je voudrais que le total s'affiche dans une zone texte et aussi si je choisis 2 ou 3 menus le total change. merci d'vance.
Code:
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<form action="envoi.php" method="post" name="form" >
<table>
 menu qte
  <select name="na">
    <option value="0">selection</option>
	<option value="1">1</option>
    <option value="2">2</option>
    <option value="3">3</option>
   </select> 
 
	 1:<select name="np[]" >
	  <option value="0">selection</option>
      <option value="g">g 1€</option>
      <option value="b">b 2€</option>
      <option value="f">f 3€</option>
      <option value="supc">h 4€</option>
      <option value="supp">ch 5€</option>
      <option value="ha">su 6€</option>
      <option value="che">sup 7€</option>
     </select>
	  boisson 1: <select name="nom[]">
	  <option>selection</option>
      <option>coca</option>
      <option>fanta</option>
      <option>sprite</option>
      <option>light</option>
      <option>minute</option>
      <option>barissart</option>
      <option>cafe</option>
    </select>
	sauce 1: <select name="s[]" onchange="calculette();" >
	  <option value="0" >selection</option>
	  <option value="ma">ma</option>
	  <option value="ket">ket</option>
	  <option value="bar">bar</option>
	  <option value="cur">cur</option>
	  <option value="bas">bas</option>
	  <option value="aig">aig</option>
	  <option value="7">sans sauce</option>
      </select>
 
	 2:<select name="np[]" onchange="calculette();" >
	  <option value="0">selection</option>
      <option value="g">g 1€</option>
      <option value="b">b 2€</option>
      <option value="f">f 3€</option>
      <option value="supc">h 4€</option>
      <option value="supp">ch 5€</option>
      <option value="ha">su 6€</option>
      <option value="che">sup 7€</option>
    </select>
	boisson 2: <select name="nom[]">
	<option>selection</option>
      <option>coca</option>
      <option>fanta</option>
      <option>sprite</option>
      <option>light</option>
      <option>minute</option>
      <option>barissart</option>
      <option>cafe</option>
    </select>
	sauce 2:<select name="s[]" >
	  <option value="0" >selection</option>
	  <option value="ma">ma</option>
	  <option value="ket">ket</option>
	  <option value="bar">bar</option>
	  <option value="cur">cur</option>
	  <option value="bas">bas</option>
	  <option value="aig">aig</option>
	  <option value="7">sans sauce</option>
      </select>
 
	3: <select name="np[]" >
	<option value="0">selection</option>
      <option value="g">g 1€</option>
      <option value="b">b 2€</option>
      <option value="f">f 3€</option>
      <option value="supc">h 4€</option>
      <option value="supp">ch 5€</option>
      <option value="ha">su 6€</option>
      <option value="che">sup 7€</option>
    </select>
	boisson 3: <select name="nom[]">
	<option>selection</option>
      <option>coca</option>
      <option>fanta</option>
      <option>sprite</option>
      <option>light</option>
      <option>minute</option>
      <option>barissart</option>
      <option>cafe</option>
    </select>
	sauce 3: <select name="s[]"  >
	  <option value="0" >selection</option>
	  <option value="ma">ma</option>
	  <option value="ket">ket</option>
	  <option value="bar">bar</option>
	  <option value="cur">cur</option>
	  <option value="bas">bas</option>
	  <option value="aig">aig</option>
	  <option value="7">sans sauce</option>
      </select>
 
  </table>
   <input name="envoyer" type="button" id="envoyer" value="Envoyer" onClick="envoie(this.form)">
      <input name="retablir" type="reset" id="retablir" value="R&eacute;tablir">
  </form>