A maker grinds a biconvex lens with a face radius of 25 cm on the first face and 40 cm on the…

Step 1. The shape. $\dfrac{1}{R_1} - \dfrac{1}{R_2} = \dfrac{1}{25\ \text{cm}} + \dfrac{1}{40\ \text{cm}} = 0.065\ \text{cm}^{-1}$.

Step 2. Flint glass. $\dfrac{1}{f} = (1.62 - 1) \times 0.065\ \text{cm}^{-1} = 0.0403\ \text{cm}^{-1}$, so $f = 24.8\ \text{cm}$.

Step 3. Crown glass. $\dfrac{1}{f} = (1.50 - 1) \times 0.065\ \text{cm}^{-1} = 0.0325\ \text{cm}^{-1}$, so $f = 30.8\ \text{cm}$.

Step 4. The power. $P = \dfrac{1}{0.248\ \text{m}} = 4.03\ \text{D}$.