The number of each type of book is what is unknown, so we can represent those quantities with variables. Let x = the number of hardbacks and y = the number of paperbacks. Then we know that: x + y = 65 (the total number of books sold) We also know the total cost of both editions, which is $1356. It can be written algebraically as: 28x + 12y = 1356 We now have a system of two equations, which can be solved by substitution. It would be easier to solve the first equation for either x or y and substitute that into the second equation.