JSFiddle - React, Tailwind, and code Playground

HTML

<table>
<tr>
<td><input id="c1" type="checkbox"/></td>
<td>342</td>
<td>24</td>
<td>Jeff</td>
</tr>
<tr>
<td><input id="c2" checked="checked" type="checkbox"/></td>
<td>343</td>
<td>29</td>
<td>Libo</td>
</tr>
</table>

JavaScript

var total = 0;
$("table tr:has(:checkbox:checked) td:nth-child(3)").each(function() {
    total += parseInt($(this).text());
});
alert(total);