SU

<table class="table table-bordered" border="1" width="35%" cellpadding="0" cellspacing="0">
    <tbody>
        <tr>
            <td class="no"><div class="blank">8</div></td>
            <td class="no"></td>
            <td class="br"></td>
            <td class="no"></td>
            <td class="no"></td>
            <td class="br"></td>
            <td class="no"></td>
            <td class="no">1</td>
            <td class="no">3</td>
        </tr>
        <tr>
            <td class="no">9</td>
            <td class="no"></td>
            <td class="br"></td>
            <td class="no"><div class="blank">1</div></td>
            <td class="no"></td>
            <td class="br"></td>
            <td class="no"></td>
            <td class="no"></td>
            <td class="no">8</td>
        </tr>
        <tr>
            <td class="bb"></td>
            <td class="bb">1</td>
            <td class="brbb">5</td>
            <td class="bb"></td>
            <td class="bb">8</td>
            <td class="brbb"></td>
            <td class="bb"></td>
            <td class="bb"></td>
            <td class="bb"></td>
        </tr>
        <tr>
            <td class="no"></td>
            <td class="no">9</td>
            <td class="br"></td>
            <td class="no"><div class="blank">8</div></td>
            <td class="no"></td>
            <td class="br">7</td>
            <td class="no">4</td>
            <td class="no"></td>
            <td class="no">2</td>
        </tr>
        <tr>
            <td class="no"></td>
            <td class="no">6</td>
            <td class="br"></td>
            <td class="no"><div class="blank">9</div></td>
            <td class="no"></td>
            <td class="br">4</td>
            <td class="no"><div class="blank">1</div></td>
            <td class="no"><div class="blank">8</div></td>
            <td class="no"></td>
        </tr>
        <tr>
            <td class="bb"></td>
            <td class="bb">8</td>
            <td class="brbb"></td>
            <td class="bb">2</td>
            <td class="bb"></td>
            <td class="brbb"></td>
            <td class="bb">7</td>
            <td class="bb">9</td>
            <td class="bb"><div class="blank"></div></td>
        </tr>
        <tr>
            <td class="no"></td>
            <td class="no"></td>
            <td class="br"></td>
            <td class="no">6</td>
            <td class="no">9</td>
            <td class="br"></td>
            <td class="no"></td>
            <td class="no">2</td>
            <td class="no"></td>
        </tr>
        <tr>
            <td class="no"></td>
            <td class="no"></td>
            <td class="br"></td>
            <td class="no"></td>
            <td class="no">1</td>
            <td class="br"></td>
            <td class="no"></td>
            <td class="no"></td>
            <td class="no"></td>
        </tr>
        <tr>
            <td class="no"></td>
            <td class="no"></td>
            <td class="br">8</td>
            <td class="no">5</td>
            <td class="no"></td>
            <td class="br"></td>
            <td class="no"></td>
            <td class="no"></td>
            <td class="no">1</td>
        </tr>
    </tbody>
</table>
<input type='button' id='hideshow' value='Show/Hide Answer'>
<script src="https://code.jquery.com/jquery-1.4.2.js"></script>
<script>
    jQuery(document).ready(function(){
        jQuery('#hideshow').live('click', function(event) {
             jQuery('.blank').toggle('show');
        });
    });
</script>
<style>
    td > .blank{
        display:none;
        color:#fff;
        font-weight: 800;
        font-size:20px;
        padding: 1px;
        background-color:red;
    }
    td.no{
        background-color: #fff;

    }
    table tr td {
        border:1px solid #ccc;
        text-align: center;
        padding: 10px;
    }
    table{
        border:2px solid #4d4d4d;
    }

    td.br{
        border-right: 2px solid #4d4d4d;


    }
    td.bb{
        border-bottom: 2px solid #4d4d4d;

    }
    td.brbb {
        border-bottom: 2px solid #4d4d4d;
        border-right:2px solid #4d4d4d;

    }
</style>