ABC Bookstore sells new books, n, for $12, and used books, u, for $8. The store earned $112 revenue last month. The store sold 4 more used books than new books. Using the substitution method, how many new and old books did ABC bookstore sell?

A. n = 4; u = 8

B. n = 8; u = 4

C. n = 8; u = 12

D. n = 12; u = 8

Let n new books and u old books were sold last year.

Revenue from new books = 12n

Revenue from old books = 8u

Last month, the store earned $112 as revenue, so we can write:

12n + 8u = 112

The store sold 4 more used books than new books, so we can write:

u = n + 4

Using the value of u, in above equation, we get:

12n + 8 (n+4) = 112

12n + 8n + 32 = 112

20n = 80

⇒

n=4

Since,

u = n + 4

u = 4 + 4 = 8

Thus, the store sold 4 new books and 8 old books last month. So the correct option is A