JSFiddle - React, Tailwind, and code Playground

HTML

<body>
    <div id="divButtons">
        <input type="button" value="1" class="btn" />
        <input type="button" value="2" class="btn" />
        <input type="button" value="3" class="btn" />
        <input type="button" value="4" class="btn" />
        <input type="button" value="5" class="btn" />
        <input type="button" value="6" class="btn" />
        <input type="button" value="7" class="btn" />
        <input type="button" value="8" class="btn" />
        <input type="button" value="9" class="btn" />
        <input type="button" value="0" class="btn" />
        <input type="button" value="Clr" class="btn" />
    </div>
    <br/>
    <br/>Item 1
    <div class="qtyBox" id="lbl060313_12341">0</div>Item 2
    <div class="qtyBox" id="lbl060313_12342">1</div>Item 3
    <div class="qtyBox" id="lbl060313_12343">2</div>Item 4
    <div class="qtyBox" id="lbl060313_12344">3</div>Item 5
    <div class="qtyBox" id="lbl060313_12345">4</div>Item 6
    <div class="qtyBox" id="lbl060313_12346">0</div>Item 7
    <div class="qtyBox" id="lbl060313_12347">1</div>Item 8
    <div class="qtyBox" id="lbl060313_12348">2</div>Item 9
    <div class="qtyBox" id="lbl060313_12349">3</div>Item 10
    <div class="qtyBox" id="lbl060313_12350">4</div>Item 11
    <div class="qtyBox" id="lbl060313_12351">0</div>Item 12
    <div class="qtyBox" id="lbl060313_12352">1</div>Item 13
    <div class="qtyBox" id="lbl060313_12353">2</div>Item 14
    <div class="qtyBox" id="lbl060313_12354">3</div>Item 15
    <div class="qtyBox" id="lbl060313_12355">4</div>Item 16
    <div class="qtyBox" id="lbl060313_12356">0</div>Item 17
    <div class="qtyBox" id="lbl060313_12357">1</div>Item 18
    <div class="qtyBox" id="lbl060313_12358">2</div>Item 19
    <div class="qtyBox" id="lbl060313_12359">3</div>Item 20
    <div class="qtyBox" id="lbl060313_12360">4</div>Item 21
    <div class="qtyBox" id="lbl060313_12361">0</div>Item 22
    <div class="qtyBox" id="lbl060313_12362">1</div>Item 23
    <div...

CSS

#divButtons {
                position: fixed;
                display: inline;
                width: 100%;
            }
            .btn {
                font-size: 22px;
                padding: 3px;
                height: 40px;
                width: 8.5%;
                display: inline;
            }
            .qtyBox {
                width: 100px;
                height: 100px;
                border: solid;
                font-size: 30px;
            }
            .Selected {
                background-color: green;
                color: yellow;
            }

JavaScript

$().ready(function () {
    var $varSelected = 'undefined';
    var $this = 'undefined';
    var varPrev = '';
    var varNew = '';

    $('.btn').click(function () {
        $this = $(this);

        if ($varSelected !== 'undefined') {
            // Get previous value
            varPrev = $varSelected.text();

            // Find value we're trying to add
            varAdding = $this.attr('value');

            if (varAdding == 'Clr') {
                varNew = '';
            } else {
                varNew = varPrev + varAdding;
            }

            // Write new value
            $varSelected.text(varNew);
        }
    });

    $('.qtyBox').click(function () {
        $this = $(this);

        // Check if we've previously had a selected box
        if ($varSelected === 'undefined') {
            // Didn't have one before -- nothing special
        } else {
            // Had one selected. We need to unselect it.
            $varSelected.removeClass('Selected');
        }

        // Select the one we have now
        $varSelected = $this;

        // Add formatting
        $this.addClass('Selected');

        // Clear value
        varNew = '';
        $this.text(varNew);
    });
});