Final answer:
The magnifying power of the objective lens being used is 40x if the total magnification of the slide is 400x and the ocular lens magnification is 10x.
Step-by-step explanation:
If the total magnification of a slide is 400x and the ocular lens has a magnification of 10x, then the magnifying power of the objective lens being used can be calculated by dividing the total magnification by the magnification of the ocular lens. Total magnification is the product of the magnification powers of the ocular and objective lenses. Therefore, the magnification of the objective lens is 40x because 400x (total magnification) divided by 10x (ocular lens magnification) equals 40x.