first, for image formation from lens
u=−60cm
f=+30cm
⇒v=u+fuf=−60+30−60×30=60cm

this real image formed by lens acts as virtual object for mirror
Real image from plane mirror is formed 20cm in front of mirror, hence at 20cm distance from lens. Now, for second refraction from lens, u=−20cm
f=+30cm
v=u+fuf=−20+30−20×30=−60cm
So, final virtual image is 60cm from lens, or 20cm behind mirror

