Donna receives a $100 gift card to MovieStop. Her Blu-ray player is able to play both Blu-rays and DVDS, so she would like to get a combination of both. Each Blu-ray cost $25.99 and DVDs cost $15.99 each. Write an equation modeling the number of Blu-rays, ​b​, and DVDs​, d,​ that Donna can purchase with her gift card.