Solve the following problem given below:
Problem: A compound microscope has a 18-cm-long barrel and an objective lens with a focal length of 8.0 mm. It is adjusted for minimum eyestrain for persons with a near point of 25 cm.
A- What is the focal length of the eyepiece to give a total magnification of 240?
I don't know how to get that answer for the above problem. Please help! Thanks.