Answer:
Yes there is, and it is 4.99.
Explanation:
The constant of proportionality (let's call it
) will relate the number of books bought
and their cost
in the following way:


For the 12 books:

And for the 21 books:

The
for the two purchases is the same, thus there is a constant of proportionality between the two purchases.
P.S:
is the price per book.