Distance between lense and film to get a focused picture

[phy112]

Homework Statement

Esperanza uses a 35-mm camera with a standard lens of focal length 50.0 mm to take a photo of her son Carlos, who is 1.3 m tall and standing 4.0 m away.

What must be the distance between the lens and the film to get a properly focused picture?
this answer is 50.6 mm.

What is the magnification of the image?
shouldnt this just be m=-q/p. i keep getting -.013 and its wrong

Homework Equations

The Attempt at a Solution

 

[Kurdt]

There doesn't seem to be anything wrong with the calculation, could it just be you need to use more significant figures?