Final answer:
To achieve a total magnification of 400X with an eyepiece magnification of 10X, an objective lens with a magnification of 40 should be used.
Step-by-step explanation:
In order to achieve a total magnification of 400X, we need to determine which objective lens to use. Given that the eyepiece has a magnification of 10X, we can use the formula:
Total Magnification = Objective Lens Magnification × Eyepiece Magnification
Substituting the known values:
400X = Objective Lens Magnification × 10X
Simplifying the equation:
Objective Lens Magnification = 400X ÷ 10X = 40
Therefore, the objective lens with a magnification of 40 should be used to view the cells under the compound microscope.