What should be the value of distance d so that final image is formed on the object itself. ${\mathrm{f}}_1=10 \mathrm{cm}$, ${\mathrm{f}}_2=20 \mathrm{cm}$

For a concave lens if the incident rays are parallel then  they will meet at focal point of concave lens 

$\therefore 20=10+\mathrm{d}\mathrm{d}=10\mathrm{cm}$