A convex lens $\mathrm{A}$ of focal length $20\mathrm{ }\mathrm{c}\mathrm{m}$ and a concave lens $\mathrm{B}$ of focal length $5\mathrm{ }\mathrm{c}\mathrm{m}$ are kept along the same axis with a distance $\mathrm{d}$ between them. If a parallel beam of light falling on $\mathrm{A}$ leaves $\mathrm{B}$ as a parallel beam, then the distance $\mathrm{d}$ (in $\mathrm{c}\mathrm{m}$) will be