JSFiddle - React, Tailwind, and code Playground

HTML

<table id="tabela">
<tr id="rowstawka">
    <td>Stawka</td>
    <td class="num">12</td>
</tr>

<tr id="rowgodziny">
    <td>Godziny</td>
    <td class="num">50</td>
</tr>

<tr id="rowPensja">
    <td>Pensja</td>
    <td>-</td>
</tr>

<tr id="rowNetto">
    <td>Pensja Netto</td>
    <td id="result">x</td>
</tr>

</table>

JavaScript

var numbers = document.querySelectorAll(".num");
var total = 1;

for (var i = 0; i < numbers.length; i++)
{
    total*= Number(numbers[i].innerText);
}

document.getElementById("result").innerText = total;