Hello Bisma, according to your query you have taken an additional subject and have got standard marks in it but have nearly passed a subject. For your information the aggregate result is calculated in the following manner:

Aggregate %age = (Sum of marks secured in all registered subjects/ Sum of total marks in all the subjects)*100

And remember no poor performance in a subject is replaced by an alternative subject as there is no such provision by any academic board. I'll give you an example, let's suppose you have 5 subjects and have got 75,78,81,83 in four, 36 in one and 95 in the higher score additional subject out of 100 marks, so according to the formula your aggregate becomes:

A = (75+78+81+83+36+95/600)*100 = 74.6%

And remember it never happens that they substitute a score with another and leave the lesser score. Rather additional subjects are there to improve overall score when best utilised. Hope it helps, thanks!