Last year the sales at a bookstore were 75% fiction books, and 25% nonfiction books.

the average cost of a fiction book was $15. the average cost of a nonfiction book was $16.

what was the average cost of all fiction and non-fiction books sold at the bookstore last year?

B = total number of books sold
F = number of fiction books sold
N = number of nonfiction book sold

B = F + N
F = .75B
N = .25B

C = average cost of all books sold
f = average cost of fiction books = 15
n = average cost of nonfiction books = 16

Total Cost = CB = Ff + Nn = F(15) + N(16)

CB = F(15) + N(16)
CB = (.75B)(15) + (.25B)(16)
CB = 11.25B + 4B
CB = 15.25B
C = 15.25

Therefore, your average cost is $15.25