KUURWA

<!DOCTYPE html>
<html>
<head>
    <style>
        table {
            border-collapse: collapse;
        }

        table, th, td {
            border: 1px solid black;
            padding: 20px;
        }
    </style>
</head>
<body>
    <table>
        <tr>
            <td>5</td>
            <td>7</td>
            <td>1</td>
            <td>4</td>
        </tr>
        <tr>
            <td>5</td>
            <td>2</td>
            <td>4</td>
            <td>1</td>
        </tr>
    </table>
    <br>
    <button onclick="sum()">Oblicz sume</button>
    <p id="sum"></p>
    <script>
        function sum()
        {
            let rows = document.getElementsByTagName("td")

            let sum = 0

            for(let row of rows)
            {
                sum += Number(row.innerHTML)
            }

            document.getElementById("sum").innerHTML = "Suma to " + sum
        }
    </script>
</body>
</html>