Let A be the number of activity books. Then the number of story books is (12-A). The "money" equation is 2A + 9*(12-A) = 45 dollars. (Counting A-books at 2 dollars and story-books at 9 dollars) Simplify and solve for A: 2A + 108 - 9A = 45 ====> -7A = 45 - 108 = - 63 ====> A == 9. Answer. Chris purchased 9 activity books and 12-9 = 3 story books.