main.js

$(document).ready(function() {

    let $randomnbr = $('.nbr'),
        $timer = 30,
        $it,
        $change, 
        $index,
        $data = 0,
        $letters = ['z', 'a', 'c', 'k', 'a', 'r', 'i', 'a' ];

        $randomnbr.each(function() {

            $change = Math.round(Math.random() * 100);
            $(this).attr('data-change', $change)
            
        });

        function random() {
            return Math.round(Math.random() *9);
        }

        function select() {
            return Math.round(Math.random() * $randomnbr.length+1);
        }

        function value() {
            $('.nbr:nth-child('+select()+')').html(''+random()+'');
            $('.nbr:nth-child('+select()+')').attr('data-number', $data);
            $data++;

            $randomnbr.each(function() {

                if(parseInt($(this).attr('data-number')) > parseInt($(this).attr('data-change'))) {
                    $index = $('.ltr').index(this);
                    $(this).html($letters[$index]);
                    $(this).removeClass('nbr');
                }
            });

        }

    it = setInterval(value, $timer );

})