Mildred and Robert are the only buyers in the market for DVDs. Mildred buys 5 DVDs when the price of a DVD is ​$6.00 ​, 4 DVDs when the price of a DVD is ​$8.00 ​, and 2 DVDs a month when the price of a DVD is ​$10.00 . Robert buys 18 DVDs a month when the price of a DVD is ​$6.00 ​, 9 DVDs when the price of a DVD is ​$8.00 ​, and zero DVDs when the price of a DVD is ​$10.00 . In the market for​ DVDs, what do we know about the quantity​ demanded?