Given parameters are
The focal length of the convex lens is f1=20cm.
The focal length of the convex mirror is, f2=20cm
The distance of the image placed from the image is, u1=30cm
At convex lens; 1/f1 = 1/v1 + 1/u1
Substituting the value of focal length and the object distance from the lens 1/20=1/di + 1/30
1/20 − 1/30 = 1/di
Where, di denotes the distance of the image at convex lens.
1/di = 1/60
At convex mirror:
1/f2 = 1/di + 1/uo
Substitutes the values of the focal length and the image distance;
1/20 = 1/60 + 1/do
1/20 − 1/60 = 1/do
Where do denote the distance of the object to the convex mirror.
1/do = 1/30
The distance of the value d; 1/d = 1/f
That is 1/d = 1/f =1/di + 1/do
1/d = 1/60 + 1/30
1/d = 90/1800
Therefore, the value of the d is 20 cm.