For the image of the overhead projector to be in focus, the distance from the projector lens to the image, \(d_{i}\) , the projector lens focal length, f, and the distance from the transparency to the projector lens, \(d_{0}\) , must satisfy the thin lens equation \(\frac{1}{f}= \frac{1}{ d_{i} }\)+ \(\frac{1}{ d_{0} }\). Which is the focal length of the projector lens if the transparency placed 4 inches from the projector lens is in focus on the screen, located 8 feet from the projector lens?

I'm especially unsure of what the last sentence means...