jquery Callback

jquery Callback

I have a script that displays data from the database unto my screen. Everything works great except I am unable to pass my jquery variable to the html code:

"fnRowCallback": function( nRow, aData, iDisplayIndex ) {
/* Append the grade to the default row class name */

var aTT = aData[4] - aData[3];
$('td:eq(4)', nRow).html( '<b>aTT</b>' );
}
What I am doing here is substracting the data element in aData[3] from aData[4] and assigning them to the variable aTT however, Im trying to display it, but on the screen it just shows the word 'aTT' as oppose to the difference between the two jquery data values.

Does anyone knows how to deal with a situation such as this, I've been searching but I have found nothing close to what Im looking for.

Thanks in advance.