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Lenses/compound microscope

Hi. Can someone please show me how to do the following problem?

"The medium-powered objective lens in a laboratory microscope has a focal length f-objective = 4.00 mm. (a) If this lens produces a lateral magnification of -40.0, whiat is the distance from the object to the objective lens? (b) What is the focal length of an eyepiece lens that will provide an overall magnification of 125?"

The answer to (a) (according to the back of the book) = 4.10 mm
and (b) = -8.0 cm

Is there an equation to figure out part a? My book just indicates that the object distance (do) for a compound microscope equals approximately the f-objective. But if that's the case, the object distance would equal 4.0 mm rather than 4.10 mm.

And for part b, I don't understand how to do the problem if they don't give you a value for the near point (N). The only equation I know regarding compound microscopes is M-total = -(di)(N) / (f-objective)(f-eyepiece). Can you figure out N if it's not given in the problem?

Thank you.