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