A publisher offers its new book in 3 formats: ebook for $3, paperback for $20, and hardcover for $30. In one hour it sold 100 copies for a total of $1080. How many paperback copies were sold?
Answer: If x, y. and z represent the respective number of copies of each format sold, then 3x + 20y + 30z = 1080 (i.e. total revenue $1080) and x + y + z = 100 (i.e., 100 copies sold) Most people would solve this system by trial and error. However, for those who have a strong mathematical background, substitute from the second equation into the first equation and solve the resulting Diophantine equation to obtain x = 60, y = 30 and z = 10. That is, 30 paperback copies were sold.
