The total magnification when using a 40x lens depends on the magnification power of the eyepiece lens you are using in combination with the objective lens.
For example, if you are using a standard eyepiece lens with a magnification of 10x, your total magnification would be calculated as follows:
Total Magnification = Objective Lens Magnification × Eyepiece Magnification
So in this case:
Total Magnification = 40x (objective lens) × 10x (eyepiece lens) = 400x
This means that you would be viewing the specimen at 400 times its actual size. If you use a different eyepiece magnification, simply substitute that into the formula to find your total magnification.