Maximum Magnification of Microscope

[JSGandora]

Homework Statement

You are building a compound microscope with an objective lens of focal length 0.70 cm and an eyepiece lens of focal length 5.0 cm. You mount the lenses 18 cm apart. What is the maximum magnification of your microscope?

Homework Equations

M=\left(\frac{25}{f_e}\right)\left(\frac{l-f_e}{d_o}\right)

The Attempt at a Solution

I tried using the above formula for overall angular magnification and got $\frac{65}{d_o}$ and so the magnification depends on $d_o$, and the smaller $d_o$ gets the larger the magnification. However it looks like the largest magnification is \infty, but the answer is 130. How do you do this problem?

 

[JSGandora]

Does anybody know how to approach this problem?

 

[JSGandora]

I feel like there is no maximum magnification unless there is a minimum distance the object can be placed to the objective lens.