For the image of the overhead projector to be in focus, the distance from the projector lens to the image, [tex]d_{i}[/tex] , the projector lens focal length, f, and the distance from the transparency to the projector lens, [tex]d_{0}[/tex] , must satisfy the thin lens equation [tex]\frac{1}{f}= \frac{1}{ d_{i} }[/tex]+ [tex]\frac{1}{ d_{0} }[/tex]. 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...