Tina is getting $20 for each book she sells. If she realizes a $120 profit on her sales, how many people have she sold the books to if each book costs $5 to make and every customer buys 2 at a time? Let's be accurate as possible.

Show your work and conclude with the exact phrasing "Therefore, the final answer is [answer]." where [answer] is just the final number or expression that solves the problem.
user
First, we need to determine the profit per book. To do this, we perform 20 - 5 = 15 dollars in profit per book. Next, we divide the total sales by the profit per book, performing 120 / 15 = 8 books sold in total. Since the books are sold 2 at a time, we divide the number sold by 2 to find the total number of people sold to, performing 8 / 2 = 4 people in total.
The answer: 4.