Camera lens problem?

Question

To focus a camera on objects at different distances, the converging lens is moved toward or away from the film, so a sharp image always falls on the film. A camera with a telephoto lens (f = 280 mm) is to be focused on an object located first at a distance of 3.71 m and then at 46.1 m. Over what distance must the lens be movable?

Answer 1

The simplest equation makes use of the "thin lens assumption", which may be iffy when applied to a telephoto lens. But here it is. From the ref, 1/FL = 1/Di + 1/Do, where FL is focal length, Do is object distance and Di is the image distance.
1/Di = 1/FL - 1/Do
Di = 1 / (1/0.28 - 1/Do)
Substitute 3.71 and 46.1 for Do in the above to get the image distances Di. The difference between the two Di values is the required range of motion of the lens. I get roughly 21 mm.
EDIT: Re answer 2, it's the distance from lens to object. Strictly speaking the user must measure object distance to the lens, not some stationary point on the camera body.

Answer 2

It's unclear if 3.71m is distance to the leans, or to the film.
The answers would be differnt of course.