how much is 249 plus service tax 12.36

This could also be  $249 + $12.36  = $261.36   ......who knows ???

You add 12.36 Percent of 249 to 249:

$${\mathtt{249}}{\mathtt{\,\small\textbf+\,}}{\mathtt{249}}{\mathtt{\,\times\,}}{\mathtt{12.36}}\% = {\mathtt{249}}{\mathtt{\,\small\textbf+\,}}{\mathtt{249}}{\mathtt{\,\times\,}}{\mathtt{0.123\: \!6}} = {\mathtt{279.776\: \!4}}$$