Since it is given after reflection and refractions. So first reflection, thence, I1 is formed 10 cm behind the mirror which acts as virtual object, now image of this virtual object from slab well be final image. So apparent distance (l1) = nr/ni(30cm + 10 cm)
= 1.5/1 x 40 cm = 60 cm
So distance b/w I1 and S = 30 cm