A symmetric biconvex lens of glass with refractive index 1.5 has a focal length of 20 cm in air…

Step 1. The radius. For a symmetric lens $\dfrac{1}{f} = \dfrac{2(n - 1)}{R}$. We want $R$, so $R = 2(n - 1)f = 2 \times 0.5 \times 20\ \text{cm} = 20\ \text{cm}$.

Step 2. One piece. The piece has a flat face and a curved face of radius $20\ \text{cm}$. $\dfrac{1}{f'} = \dfrac{n - 1}{R} = \dfrac{0.5}{20\ \text{cm}}$, so $f' = 40.0\ \text{cm}$, twice the focal length of the whole lens.

Step 3. The mirror. The light crosses the piece twice and the flat mirror has no power: $\dfrac{1}{F} = \dfrac{2}{f'} = \dfrac{2}{40.0\ \text{cm}}$. So $F = 20.0\ \text{cm}$.

Step 4. The image. $\dfrac{1}{v} = \dfrac{1}{20.0\ \text{cm}} - \dfrac{1}{60\ \text{cm}} = \dfrac{1}{30\ \text{cm}}$, so $v = 30.0\ \text{cm}$ in front. The magnification is $m = -\dfrac{30.0\ \text{cm}}{60\ \text{cm}} = -0.500$.