if your eyepiece has a magnification of 10x and you are viewing through an objective lense of 20x, you multiply the magnifications, equalling a total magnification of 200x.
The compound microscope uses two lenses at once: the eye-piece lens and one of the objective lenses. The magnification of the microscope is the product of the magnifying power of these two lenses. This sounds complicated but it is very easy to calculate because the magnification is written on each of the lenses.
