An older camera has a lens with a focal length of 60 mm and uses 36-mm-wide film to record its images.
Using this camera, a photographer takes a picture of the Golden Gate Bridge that completely spans the width of the film.
Now he wants to take a picture of the bridge using his digital camera with its 10-mm-wide CCD detector.
What focal length should this camera's lens have for the image of the bridge to cover the entire detector?
Express your answer to two significant figures and include the appropriate units.
