A thin convex lens of focal length $20\ \text{cm}$ and a thin concave lens of focal length $10\ \text{cm}$ are placed on the same axis, $5\ \text{cm}$ apart, as shown in the figure. The focal length of the combination is

Fig. 1: A convex lens of focal length 20 cm and a concave lens of focal length 10 cm on the same axis, 5 cm apart.