How to update Brutto based off of VAT Netto and ilosc?

Question

I want to update kwota brutto, wartosc netto and wartosc brutto based off of ilosc kwota netto and vat but it doesn't seem to work. I literally have no clue what casues it to not work. I need it to update everytime someone clicks VAT frop down list and picks vat(tax) value. I want it to be 100% javascript because i don't want to use something i don't understand JS:

function brutto(){
  let tab = document.getElementById('tab');
  let rows = document.getElementById('tab').rows.length;
  let netto = document.getElementsByClassName('netto');
  let ilosc = document.getElementsByClassName('ilosc');
  let vat = document.getElementsByClassName('vat');
  let brutto = document.getElementsByClassName('brut');
  let warnet = document.getElementsByClassName('warnet');
  let warbrut = document.getElementsByClassName('warbrut');
  for (var i=0;i < tab.length;i++){
    let net = netto[i].value;
    let vatt =vat[i].value;
    if (vatt==="ZW" || vatt==='NP' || vatt==='0%'){
      vatt =100;
    }else if (vatt==='3%'){
      vatt=103;
    }else if(vatt==='8%'){
      vatt = 108;
    }else if (vatt==='23%'){
      vatt= 123;
    } else brut.innerHTML='';
    let il = ilosc[i].value;

    brutto[i].innerHTML = ((net/100)*vatt).toFixed(2);
    console.log(brut);
  }
}

<div class="Lewy">
  <div id="menu"><p>Menu</p>
    <ul id="linki">
      <li><a href="index.html">Strona Główna</a></li>
      <li><a href="rkhtml.html">Różne kontrolki HTML</a></li>
      <li><a href="tprac.html">Tabela Pracowników</a></li>
      <li><a href="fVAT.html">Tabela Faktur VAT</a></li>
      <li><a href="del.html">Tabela Delegacji BD</a></li>
      <li><a href="forum.php">Dane Kontrahentów</a></li>
    </ul>
  </div>
</div>
<div class="Prawy">
  <table id="tab">
    <tr>
      <th> Lp.   </th>
      <th> Opis   </th>
      <th> MPK   </th>
      <th> Kwota Netto   </th>
      <th> Ilość   </th>
      <th> VAT   </th>
      <th> Kwota brutto </th>
      <th> Wartość Netto  </th>
      <th> Wartość Brutto   </th>
    </tr>
    <tr>
      <td>1.</td>
      <td> Kupno mebli w postaci biurka.</td>
      <td>Dział IT</td>
      <td class="netto">456</td>
      <td class="ilosc"> 3</td>
      <td><select class="vat" onclick="brutto()">
        <option disabled selected value></option>
        <option>ZW</option>
        <option>NP</option>
        <option>0%</option>
        <option>3%</option>
        <option>8%</option>
        <option>23%</option>
      </select></td>
      <td class="brut"></td>
      <td class="warnet"></td>
      <td class="warbrut"></td>
    </tr>
    <tr>
      <td>2.</td>
      <td> Kupno mebli w postaci biurka.</td>
      <td>Dział IT</td>
      <td class="netto">381</td>
      <td class="ilosc"> 8</td>
      <td><select class="vat" onclick="brutto()">
        <option disabled selected value></option>
        <option>ZW</option>
        <option>NP</option>
        <option>0%</option>
        <option>3%</option>
        <option>8%</option>
        <option>23%</option>
      </select></td>
      <td class="brut"></td>
      <td class="warnet"></td>
      <td class="warbrut"></td>
    </tr>
    <tr>
      <td>3.</td>
      <td> Kupno mebli w postaci biurka.</td>
      <td>Dział IT</td>
      <td class="netto">100</td>
      <td class="ilosc"> 5</td>
      <td><select class="vat" onclick="brutto()">
        <option disabled selected value></option>
        <option>ZW</option>
        <option>NP</option>
        <option>0%</option>
        <option>3%</option>
        <option>8%</option>
        <option>23%</option>
      </select></td>
      <td class="brut"></td>
      <td class="warnet"></td>
      <td class="warbrut"></td>
    </tr>
    <tr>
      <td>4.</td>
      <td> Kupno mebli w postaci biurka.</td>
      <td>Dział IT</td>
      <td class="netto">4562</td>
      <td class="ilosc"> 2</td>
      <td><select class="vat" onclick="brutto()">
        <option disabled selected value></option>
        <option>ZW</option>
        <option>NP</option>
        <option>0%</option>
        <option>3%</option>
        <option>8%</option>
        <option>23%</option>
      </select></td>
      <td class="brut"></td>
      <td class="warnet"></td>
      <td class="warbrut"></td>
    </tr>
    <tr>
      <td>5.</td>
      <td> Kupno mebli w postaci biurka.</td>
      <td>Dział IT</td>
      <td class="netto">900</td>
      <td class="ilosc"> 12</td>
      <td><select class="vat" onclick="brutto()">
        <option disabled selected value></option>
        <option>ZW</option>
        <option>NP</option>
        <option>0%</option>
        <option>3%</option>
        <option>8%</option>
        <option>23%</option>
      </select></td>
      <td class="brut"></td>
      <td class="warnet"></td>
      <td class="warbrut"></td>
    </tr>

  </table>
</div>