Math

Guess #1: 3 science fiction novels and 1 mystery novel

Find the number of books:

3 + 1 = 4

Find the total cost in dollars:

(9 × 3) + (2 × 1) = 27 + 2 = 29

The guess is incorrect. Try another guess.

Guess #2: 2 science fiction novels and 2 mystery novels

Find the number of books:

2 + 2 = 4

Find the total cost in dollars:

(9 × 2) + (2 × 2) = 18 + 4 = 22

The guess is incorrect. Try another guess.

Guess #3: 1 science fiction novel and 3 mystery novels

Find the number of books:

1 + 3 = 4

Find the total cost in dollars:

(9 × 1) + (2 × 3) = 9 + 6 = 15

The guess is correct. Uncle Porter bought 1 science fiction novel and 3 mystery novels.

Easy

Let \(s\) be the number of science fiction, and \(m\) be the number of mystery novels. We can form 2 equations: \(9s + 2m = 15, s + m = 4\). Solving those 2 equations gives \(s = 1, m = 3\)