Distance from the object to objective lens and focal length

[bubothedog]

Homework Statement

1) The medium-power objective lens in a laboratory microscope has a focal length fobjective = 3.70mm . If this lens produces a lateral magnification of -42.5, what is its "working distance"; that is, what is the distance from the object to the objective lens?
2)
What is the focal length of an eyepiece lens that will provide an overall magnification of -130? Assume student's near-point distance is N = 25 cm

Homework Equations

m=-di/f_objective

The Attempt at a Solution

I tried using that formula to solve for this distance for object but it doesn't work out. I got that formula from a book where they assume that do is approx to be f_objective. Tbh, I don't quite understand how to get this "working distance". Anyone can help?

 

[Matternot]

Have you drawn a ray diagram? It should help you understand how "m=-di/fobjective" can be derived and thus, help you understand the question.