The best five subjects will be considered for calculation of percentage.

So the fifth subject would be one in which you get higher marks.  

Hello,
Let us keep the top 4 subjects on one side.Then your 5th subject will either Hindi or PHE.From both the subjects,the subkect in which your marks will be more in board exam then that subject would be used in calculating percentage of class 12th board exams.
Good luck.

Percentage in class12th js calculated on basis of best of Four subjects.

This shall require one core subjects and rest three drom List A subjects.

To know which 4 subjects are taken for calculation according tobstream is sharedbelow.
https://www.google.co.in/amp/s/www.news18.com/amp/news/india/delhi-university-admissions-how-to-calculate-best-of-4-from-cbse-results-1414615.html

The percentage would be calculated as per best of four subjects. But in some cases percentage of main five subjects is considered.