Add a simple listener

HTML

<!-- Learn about this code on MDN: https://developer.mozilla.org/en-US/docs/Web/API/EventTarget/addEventListener -->
<h1> CALCULATEUR OPTIMISEUR
DE DON AUX ASSOCIATIONS </h1>
<div id="tax-input">
<h3>Impots Payés: </h3>
<label>Montant :</label>
<input id="tax-value" type='number'>
</div>

<div id="output">
<h3>Vous pouvez donc faire un </h3>
<h4><span id="m"></span> € de Don Total</h4>
Réparti en  
<br/>
<span id="f"></span> € de Don Financier 
<br/>
+
<br/>
<span id="w"></span> € de Don en équivalent Compétences/Moyens<br/>

</div> 

<div id="work-rate">
<h1>Calcul du taux horaire</h1>
<select name="Salaire" id="hourly-rate-by-job">
  <option value="unset">Choisir un job</option>
  <option value="10">Job 1 (10 €/heure)</option>
  <option value="15">Job 2 (15 €/heure)</option>
  <option value="20">Job 3 (20 €/heure)</option>
</select>

<br/>
<label for="use-manual-rate">Saisir le salaire horaire: </label>
<input id='use-manual-rate' type='checkbox'>
<br/>
<label>salaire horaire :</label>
<input id="hourly-rate-manual" type='number'>
<br/>
Taux horaire calculé: <span id="hourly-rate"></span> €<br/>

Nombre d'heures à effectuer : <span id="wh"></span> h<br/>
</div>

JavaScript

var workGift; //Global variable to track the donated work budget.

function calculateGift() {
	var taxVal = parseFloat(document.getElementById("tax-value").value);
  var m = taxVal / 0.66;
  var f = m * 0.66;
  var w = m - f;
  
  document.getElementById("m").innerHTML  = Math.round(m *100)/100;
  document.getElementById("f").innerHTML  = Math.round(f*100)/100;
  document.getElementById("w").innerHTML  = Math.round(w*100)/100;
  workGift = w;
}

function activateRateByJob (){
	document.getElementById('hourly-rate-manual').disabled = true;
  document.getElementById('hourly-rate-by-job').disabled = false;
  document.getElementById('use-manual-rate').checked = false;
}

function activateManualRate (){
	document.getElementById('hourly-rate-manual').disabled = false;
  document.getElementById('hourly-rate-by-job').disabled = true;
  document.getElementById('use-manual-rate').checked = true;
}

function calculateWagesAndWorkhous(hourlyRate) {
	var HourlyWages = hourlyRate * 1.75 * 1.15
  var hours = workGift / HourlyWages
  document.getElementById("hourly-rate").innerHTML  = Math.round(HourlyWages * 100) / 100;
  document.getElementById("wh").innerHTML = Math.round(hours * 10) / 10;
}

// add event listener to table
var txel = document.getElementById("tax-value");

txel.addEventListener('change', calculateGift , false);


var hrjob_el = document.getElementById("hourly-rate-by-job");
hrjob_el.addEventListener('click',activateRateByJob  , false);
hrjob_el.addEventListener('change',function(){calculateWagesAndWorkhous(this.value);}  , false);

var hrman_el = document.getElementById("hourly-rate-manual");
hrman_el.addEventListener('click', activateManualRate  , false);
hrman_el.addEventListener('change',function(){calculateWagesAndWorkhous(this.value);}  , false);
var usemanrate_el = document.getElementById('use-manual-rate');
usemanrate_el.addEventListener('click', function() {
	if(this.checked){
  	activateManualRate();
  }
  else
  {
  	activateRateByJob();
  }
}  ,...