JSFiddle - React, Tailwind, and code Playground

HTML

<table id="tabela">


<tfoot>
<tr><td colspan="2"></td><td align="right">Total: <span id="total" class="total"></span> </td></tr></tfoot>

<tbody><tr>
<td><input type="checkbox" value="16" class="price">
item A (16)</td><td><select value="" class="qty" name="qty">
<option value="1">1 unidade</option>
<option value="2">2 unidades</option>
<option value="3">3 unidades</option>
<option value="4">4 unidades</option>
</select></td>
<td align="center"><span id="amount" class="amount">0</span></td></tr>

<tr>
<td><input type="checkbox" value="45" class="price">
item B (45)</td><td><select value="" class="qty" name="qty">
<option value="1">1 unidade</option>
<option value="2">2 unidades</option>
<option value="3">3 unidades</option>
<option value="4">4 unidades</option>
</select></td>
<td align="center"><span id="amount" class="amount">0</span></td></tr>

<tr>
<td><input type="checkbox" value="61" class="price">
item C (61)</td><td><select value="" class="qty" name="qty">
<option value="1">1 unidade</option>
<option value="2">2 unidades</option>
<option value="3">3 unidades</option>
<option value="4">4 unidades</option>
</select></td>
<td align="center"><span id="amount" class="amount">0</span></td></tr>


</tbody></table>

JavaScript

$(document).ready(function(){

	update_amounts();
    $('.qty').change(function() {
	update_amounts();
    });
});


function update_amounts()
{
    var sum = 0;
    $('#tabela > tbody  > tr').each(function() {
        var qty = $(this).find('option:selected').val();
        var price = $(this).find('.price').val();
        var amount = (qty*price)
        sum+=amount;
        $(this).find('.amount').text(''+amount);
    });

    $('.total').text(sum);
}