A converging beam of rays is incident on a diverging lens. Having passed through

The incident converging beam forms a virtual object for the diverging lens. Without the lens, the rays meet at a point 5 cm closer to the lens than with the lens. With lens, the rays meet at 15 cm on the opposite side (real image, v = +15 cm). Without lens, the convergence point is 10 cm from the lens (object distance, u = -10 cm, negative because virtual object). Using the lens formula 1/f = 1/u + 1/v with proper sign convention (u negative for virtual object, v positive for real image, f negative for diverging lens): 1/f = 1/(-10) + 1/15 = -1/10 + 1/15 = -1/30, so f = -30 cm.