[Dilettante_Pro]

<div class="p">Первый</div>
<div class="p">Второй</div>
<div class="p">Третий</div>
<div class="p">Четвертый</div>
<button class="btn">Sum</button>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.4/jquery.min.js"></script>

<script>
var num = $('.p'),
        btn = $('.btn');

    btn.on({
        'click': sum
    });

    function sum() {
        for (var i = 0; i < num.length; i++) {
            $(num[i]).html( $(num[i]).html() + ' ' + (i + 1));
        }
    }
</script>