A microscope with a tube length of 180 mm achieves a total m

A microscope with a tube length of 180 mm achieves a total magnification of 400× with a 20× objective and a 20× eyepiece. The microscope is focused for viewing with a relaxed eye.

A. How far is the sample from the objective lens?

I tried 9.5 mm ( but that wrong )

Solution

The overall magnification would be:

M = 20*(2L-20P)/P

where P is the distance and L the tube length

so:

400P = 40L - 400P

=> 800P = 40*180

=> P = 9 mm

Finally we get: