Final answer:
The image of Sally's face will appear to be 54 cm behind the plane mirror, which is the same distance her face is in front of it, due to the Law of Reflection.
Step-by-step explanation:
The image of Sally's face in a plane mirror will be located the same distance behind the mirror as her face is in front of it. Therefore, if her face is 54 cm in front of the mirror, the image will appear to be 54 cm behind the mirror. This is due to the Law of Reflection, which states that the angle of incidence equals the angle of reflection. Images in a plane mirror are virtual, meaning they cannot be projected on a screen because they appear to be behind the mirror at the same distance as the object is in front.