Pose a system of equations to find the number of new books that have been sold. Calculate how many books were sold with a 50% discount.
In an online shop, 800 copies of a textbook have been sold, including new and used ones, and a total of 7110 euros has been obtained. A new books costs 10 euros, while the used ones are sold at a discount of 40% or 50% depending on their condition. It has been proven that for every three new books a used one has been sold.