From lens formula for convex lens, we have
f1=v1−u1
⇒301=v1+601
⇒601=v1
Applying lens formula to concave lens, we have
f1=v1−u1
⇒−1201=v1−401,(∵u=+40cm)
⇒v=60cm
Virtual object 10 cm behind plane mirror.
Hence real image 10 cm in front of mirror.
Hence 60 cm from convex lens.