Final answer:
To see the back of your head using mirrors, you would need two mirrors. This arrangement of two mirrors will allow you to see the back of your head by reflecting the light rays multiple times.
Step-by-step explanation:
The question is not related to red reflective triangles, as mentioned in the subject line. However, the question provided in the body is about devising an arrangement of mirrors to see the back of your head and the minimum number of mirrors needed for this task.
To see the back of your head using mirrors, you would need two mirrors. Place one mirror in front of you, perpendicular to your line of sight, and the second mirror behind you at an angle so that it reflects the image from the first mirror back to you.
This arrangement of two mirrors will allow you to see the back of your head by reflecting the light rays multiple times.