First find the number of mysteries by multiplying 85 books by 20%:
80 x 0.20 = 17
Now add mysteries and adventure books together and subtract from 85 to find the number of fantasy books:
17 + 18 = 35
85 - 35 = 50
So 50 out of 85 books are fantasy.
This is written as 50/85 which simplifies to 10/17.
