I'm really stuck with this question and I have no idea how to start.

Q. A compound microscope has an objective lens with a focal length of 5 cm and an eyepiece with a focal length of 1 cm. If its magnification is 100X then the tube length is:

A 12 cm
B 0.05 cm
C 20 cm
D 6 cm
E 10 cm

I really need help in working out this question, please.

This site should provide all of the information you want:

Microscope

Thanks a lot for that!