Alexa owns a total of 45 fiction and nonfiction books. If the number of fiction books is 15 more than 4 times the number of nonfiction books, how many fiction books does she own?

A) 42 B) 39

C) 36 D) 33

Let's call x the number of fiction books and y the number of nonfiction book

x+y=45

x=4y+15

solve by substitution, you know the x value, plug it into the first equation.

(4y+15) + y=45

5y+15 = 45

5y=30

y=6

use y to solve for x

x+6=45

x=41

alexa has 41 fiction books and 6 nonfiction books