It might be c or d if not then sorry
For the cost to be the same:
18648+23.5b=83556+10b subtract 10b from both sides
18648+13.5b=83556 subtract 18648 from both sides
13.5b=64908 divide both sides by 13.5
b=4808
So if 4808 books are produced the cost of both methods is the same.
Step-by-step explanation:5 times 2=10 but 6 times 2=12 not 15 so 5/6 is proportional to 10/12 not 10/15