Question

How far should and object be from a concave sperical mirror of radius 36cm to form a real image one-ninth it's size?

Answer 1

Let the object distance be Do. The image distance must be Di = Do/9 to achieve the desired magnification.

The focal length of the mirror is
f = R/2 = 18 cm

Solve the basic equation
1/Do + 1/Di = 1/f

It isn't hard.

Answer 2

180

Answer 3

90