How is total magnification for an objective determined for your microscope?

The standard microscope is that the eyepiece is 10x magnification, and three types of powered magnification helps it magnify even more. Low power is 4x, Medium power is 10x, and High power is 40x. Eyepiece and Low power is 40x, Eyepiece and Medium power is 100x, and Eyepiece and High power is 400x magnification in revolance to the naked eye.