Question

if i get low marks in any one subject, the low marks replace with 6 aditional subject marks class 10 cbse 2018?

Answer 1

I guess nooo
But nit sure

Answer 2

Yes it will be if you are appearing for the 6th subject as well.
But if you get low marks in any of the main 5 subjects then, then %age will not be totally replaced or covered up but partially. 
So try doing your level best and think big!!
BEST OF LUCK FOR MATHS AND SST!!