As image formed is real, therefore lens must be convex, v=20cm.
Let f1, be the focal length for this lens and f2 be the focal length of the other lens put in contact later.
for the first lens,
∴1f1=1v1−1u1=120−1u1-----------------(1)
After placing it in contact with another lens, the image shifted to 10cm towards the combination
i.e.,
v2=(20−10) cm=10 cmLet
fnet be the effective focal length of the combination, which can be given as:
∴1fnet=1f1+1f2=1v2−1u2=110−1u1