index.html.erb

<div class="grid-container">
    <div class="grid-x grid-margin-x margin-vertical-2">
        <div class="cell large-12">

            <table>
                <tbody>
                    <tr>
                        <td>Manzanas</td>
                        <td>5</td>
                    </tr>    
                    <tr>        
                        <td>Pera</td>
                        <td>7</td>
                    </tr>
                    <tr>                    
                        <td>Uva</td>
                        <td>4</td>
                    </tr>
                    <tr>
                        <td>Total</td>
                        <td id="total"></td>
                    </tr>
                </tbody>
            </table>

        </div>
    </div>
</div>

<script>
    var a = document.querySelectorAll("table > tbody > tr");
    var suma = 0;
    for(var i = 0; i<a.length; i++) {
        var elem = a[i];
        suma += parseInt(elem.querySelector("td:nth-child(2)").textContent);
    }

    document.getElementById('total').value = parseInt(suma); 
</script>