A simple microscope is rated at 5 X for a normal relaxed eye....

The magnifying power of a simple microscope for a relaxed eye (image at infinity) is given by $M = \dfrac{D}{f}$, where $D$ is the near point of the observer's eye and $f$ is the focal length of the lens.

For a normal eye, the near point is $D = 25 \text{ cm}$. Given that the magnifying power is $5 \text{ X}$, we have:

$5 = \dfrac{25}{f} \Rightarrow f = 5 \text{ cm}$

For a farsighted eye with a near point $D' = 40 \text{ cm}$, the magnifying power for a relaxed eye will be:

$M' = \dfrac{D'}{f} = \dfrac{40}{5} = 8$

Thus, the magnifying power is $8 \text{ X}$.