- A girl shines a flashlight on a mirror
- A flashlight emits light
- Therefore, when a girl shines a flashlight on a mirror, light will shine on the mirror

- When a girl shines a flashlight on a mirror, light will shine on the mirror
- A mirror reflects light
- So, when a girl shines a flashlight on a mirror, the light will be reflected by the mirror

- When a girl shines a flashlight on a mirror, the light will be reflected by the mirror
- If an object reflects light toward the eye then that object can be seen
- So, if the mirror reflects light toward another girl's eye, then the other girl will see the light

Therefore, if the mirror reflects light toward another girl's eye, then the other girl will see the light
Question
A girl signals her friend by shining a flashlight on a mirror. Her friend can see the signal because

Context
- A mirror reflects light 
- A girl shines a flashlight on a mirror 
- If an object reflects light toward the eye then that object can be seen 
- A flashlight emits light