He has 286 Fiction Books.
Why?
Assume number of Non Fiction Books to be x
So, number of Fiction Books will be 11x
The sum of both the books is 312
So, 11x + x= 312
12x=312
x=312÷13=26
x was the number of Non Fiction Books. So, there are 26 Non Fiction Books.
Fiction Books= 11x=11*26=286.
Hence, there will be 286 Fiction Books in Mark's bookcases.