Sure you got it right.....

                                              Its 85......

                                Probably your first thought was about 87!!!

                                                       But what I thought was about reverse mathematics....

                                                                    Subtracted the number from 100

                                                           Now its going to get very tough....

                                                                USE YOUR BRAINS WELL

CLUE:

          As my birthday approaches I start to collect leaves - a little bizarre perhaps, but I enjoy it! On the first day of the month I collect one leaf, on the second day I collect two and so on. So by my birthday I will have collected 276 leaves altogether. On which day of the month is my birthday?

                                                                                                  15