The lens of an eye is 2.5 cm from the retina…

Step 1. The focal length. $\dfrac{1}{f} = \dfrac{1}{2.5\ \text{cm}} + \dfrac{1}{40\ \text{cm}} = \dfrac{16 + 1}{40\ \text{cm}} = \dfrac{17}{40\ \text{cm}}$, so $f = 2.35\ \text{cm}$.

Step 2. The power. $P = \dfrac{1}{0.0235\ \text{m}} = 42.5\ \text{D}$.

Step 3. The extra. $42.5\ \text{D} - 40\ \text{D} = 2.5\ \text{D}$, which is $\dfrac{1}{0.40\ \text{m}}$.